Terry Prattchet did this really well with gnomes. 

 The problem is this: there are many jobs in the city for which a gnome is equally (or in some cases more) competent, such as rat-catching (as detailed above) and watch-making, as their small hands make them ideal for performing intricate work. As money is generally paid for the amount of work done, gnomes are invariably paid the same for doing these jobs as a human or dwarf would. However, as a gnome is much smaller than a human or a dwarf, the money will stretch further. For example, a dollar might buy a loaf of bread. To a human or dwarf, that is two or three meals. To a gnome, the same loaf of bread is food for a week and can also be hollowed out to provide emergency accommodation.

https://discworld.fandom.com/wiki/Gnomes

When I first watched the movie it occurred to me that the rodents must have a huge financial advantage over larger animals

Being a mob boss probably helped, too!

Frankly, except for deconstructing harmful stereotyping, legitimate physical differences between species got aggressively downplayed in that movie.

The details about falling assumes Earth level gravity too. That might be different enough to change the scale of things. When gravity is especially high you aren’t likely to evolve very tall creatures because vertical movement becomes punishingly expensive.

For massive size differences I would expect that to affect brain size and intelligence. Below a minimum size you’re not likely to get higher intelligence because the brain just isn’t complex enough.

There’s also interesting problems with heat. Larger bodies have less surface area to dissipate heat and so are more prone to overheating. Tiny bodies may lose heat to their environment too easily.