@Background Pony #887C
So it stretches to the right size then only contracts a bit when released? That could maybe work?

@Draco Dei
Maybe a ductile material with high maximum tensile force and work hardening could work?

No need to play only on the elastic behaviour.

@Background Pony #BF5D
Yeah, that is an issue. Also, it must be harder to breath in that unless magic can create constant-force stretch matterial that requires no more force applied to stretch the first tenth-of-a-hoof-length than the fifth tenth-of-a-hoof-length. A properly fitted suit would only be the same percentage smaller than the wearer in every area as one fitted for Zipp. Well, maybe a LITTLE variation due to materials-science differences between muscle and fat, but either way, not only Rarity, but Twilight (who knows materials science), and Dash (who knows stream-lining suits) are looking down from Skyros and shaking their heads. Well, unless, as I said, magical properties that violate the spring equation.

Simple version:
Force Needed = (A Constant Value Specific to the Specific Spring) × (Distance Stretched From Unstretched Length).

More detailed version:
https://en.wikipedia.org/wiki/Hooke's_law

Poor FatPipp. Hard to fit into a human suit.