Don't rodents, and infact most Gilres, really enjoy being in tight confined spaces? Like, that comforts/relaxes them?

Same reason bunnies are totally chill hanging out inside the fake bottom of a magicians hat.

Ornithologists do something similar when they have to handle a small bird in the field or in a lab: the ice cream cone grip

The bags are often used to administer injections or decapitate a mouse or rat in lab settings iirc

This is all I can see:

Do they make human-sized ones?

The DecapiCone:

Make injections and decapitation quicker and easier with Braintree Scientific's DecapiCones. Tapered plastic film tubes provide quick and easy restraint of rats, mice, and other small animals. I.P. injections can be made directly through the film! DecapiCones restrain post-decapitation kicking and prevent personal contact with feces or urine. A unique dispenser holds DecapiCones open and ready for use. Simply hold the DecapiCone in one hand and introduce the animal with the other. Animals enter readily, heading for the breathing hole at the small end. Roll and squeeze the large end closed. They may be used repeatedly for injections and simply discarded when soiled. For decapitation, hold at the rear and insert the small end into the decapitator.

Hilarious.

The first sentence had me thinking this was a 'Gen-Z doesn't know PCs' thing.

It got so much better.

The last time I saw this meme, someone commented that these bags are made and used so that the mouse can be euthanized after an experiment. Supposedly this holds the mouse in place so that it can be "cleanly" decapitated.

Would be cool if it would eat bugs and you could take it around your house and hoover up all the insects you find.