Sure thing! Essentially, the Imperial Roman Army was a much more 'refined' machine than the army of the mid-Republic.

In the mid-Republic, what effectively happens every time the Republic needs to raise new troops is they send out a call for the citizenry of the city of Rome to line up according to their class, magistrates pick a bunch of young men who are already armed and trained (as militia, by their fathers and community, as is traditional), and then decide what to do with them. Typically they'd put them through some additional drilling and training, but most of the work is already done, and the troops are largely used as-is - it is a citizen levy in the truest sense, calling up troops from the citizenry as-needed. Not only that, but the nature of the wars of the mid-Republic meant that there remained a sense of danger - exceptional measures like enfranchising non-citizens could be resorted to if there was a disaster because all the voters would be suitably terrified of having their lands plundered and their families murdered if the war went badly.

Effectively, then, calling up more troops has a distinct political cost, but with the constantly changing fortunes of elected politicians, as long as the war seems sufficiently important (or can be spun as such to the electorate), the Republic can continue to call up troops until it collapses entirely. During the Second Punic War, it's estimated that ~20% of the adult male citizen population of Rome was killed. By comparison, during WW1, around ~15% of the adult male population of the major belligerents died before the will to keep fighting collapsed in the Central Powers.

During the Principate, the period of the Roman Empire as most people think of it, the Legions were a largely volunteer force. Rome doesn't say "Citizens of the Eternal City, come to the Field of Mars to be chosen for military service!" That's not how it works anymore. If the Empire needs more troops, it largely has to convince people to volunteer (with a few exceptional incidents of conscription excluded). And furthermore, much of the recruitable population is scattered across the Empire in Roman colonies, instead of ~90% of it being in or within a few days' march of the city of Rome itself. So that means organizing new recruitment drives is a much more involved, expensive, and time-consuming endeavor. You've got citizen colonies everywhere from Spain to Africa to the Levant to draw on, sure, but getting a fresh batch of recruits from each of them takes much more time than a week and a few dozen mounted messengers.

Not only that, but the whole "Citizen farmer trained to fight by his dad" is not only pretty dead amongst the working class of the Empire, but is in any case insufficient for the highly refined military machine of the Principate. Now legionaries are put through a battery of checks - they must be the proper height and health, they must be of good moral character, and so on. After that, they're intensely drilled and trained over a matter of months, and Roman discipline is a distinctly unpleasant experience which most people will not enjoy. If they get through all that, then they are ready to be legionaries - a system which resembles modern military training more than "Call up the citizens and hope they know which end of a sword is which".

Now, that means that the Legions of the Principate are an intensely menacing machine - you very rarely see them lose pitched battles except against the Persians (they never really managed to figure out how to decisively counter Persian armies), and their additional utility in things like engineering and policing duties is also a major advantage. But it also means that any losses sustained are difficult to replace - prospective recruits must be found, they must be convinced with a mixture of persuasion and the promise of monetary gain, and then they must be filtered and trained to maintain the overall excellence of the Legions.

Additionally, only Roman citizens were permitted to enter the Roman Legions, so much of the territory that Rome had absorbed could not immediately contribute to the Legions - only after a few generations do citizen populations in the province become large recruitment pools for the Legions. It's roughly estimated that half of the Roman Empire's military forces were legionaries, and half were 'auxilia', provincially-recruited troops who were awarded citizenship after their term of service. Non-citizens outnumbered citizens to a greater degree than this, but the tendency was to keep the prestigious citizen Legions around the same size as the provincial Auxilia, presumably to maintain the military dominance of the citizenry over provincials. The Auxilia were put through pretty similar training and recruitment standards, though conscription was used regularly in addition to volunteer recruitment.

So basically, the mid-Republic could pull additional troops out of its ass anytime it needed to, because the 'low-quality' (compared to the Principate; Roman infantry was always well-regarded) troops were already 'there' and felt like if they didn't fight, they and their families would be killed anyway by the enemy; while the Empire needed time to replenish troops, because it needed to find them, convince them, train them to a higher standard, and keep only men who were highly motivated to serve a ~20 year term under military discipline even while being posted on some godless and unpleasant frontier of the Empire stomping barbarians to protect provincials they had no connection to.

And if things go wrong, if there's a disaster in the Principate period, what happens is "Everyone back at Rome wonders if it's really worth bringing Civilization(tm) to these far-off savage frontiers and starts asking the people in charge very uncomfortable questions about where their sons and tax money are going" instead of the more mid-Republic reaction of "Holy shit, everyone is now donating their jewelry and we're going to give everyone we can find a sword so Rome doesn't end up sacked and enslaved"