This kind of went over my head, what was up with our shield in the last game? That’s something I don’t know that I actively noticed.
Perfect shielding in the past required you to shield an attack right as it was about to hit you (hitting shield one or two frames before getting hit I think). If you imagine a projectile being shot towards you, you'd have to block it at the last second.
If you had a large shield, it was easier to perfect shield because when you put up your shield it covers more space, meaning there's a larger zone around you that you can perfect shield in. Imagine the extreme case where your shield is as large as the entire stage. If you press shield any time there's an attack out on the stage you'll perfect shield because you had your shield hit within the first frame or two of putting it up.
Yoshi's shield is tiny though, and basically just covers his body. This means that to perfect shield, the attack basically had to already be on top of you. Other characters with shields larger than their bodies can put up their shields when attacks are further away and still get the perfect shield. Even worse, because Yoshi's nose is so big, there's always the possibility of getting beaned in the nose before even shielding.
Since perfect shielding now happens on RELEASING the shield, it doesn't really matter how small our shield is because everyone is already shielding, so everyone has to let go right before their shield gets hit. I suppose they get a slight advantage, since an attack that would hit their shield might just miss them if they release their shield, but that's kind of an edge case since shields shrink over time so their shields shouldn't be much larger than their hurtboxes anyways.