#Generally, yes

The equine weakness only prevents you from wearing armor that goes on the legs. Pants are not, generally speaking, armor. And the general rule for magic items is

In most cases, a magic item that's meant to be worn can fit a creature regardless of size or build. Many magic garments are made to be easily adjustable, or they magically adjust themselves to the wearer.

So generally speaking, magic pants should resize to fit a centaur, but there may be exceptions. Use Magic Device is unnecessary.

In fact, the description continues,

Rare exceptions exist. If the story suggests a good reason for an item to fit only creatures of a certain size or shape, you can rule that it doesn't adjust.

but notice that this is something special about the item—some good reason why a particular item wouldn’t adjust. Not something special about your body. The rules say that typical magic items adjust for any body size or shape.

Generally, yes

The equine weakness only prevents you from wearing armor that goes on the legs. Pants are not, generally speaking, armor. And the general rule for magic items is

In most cases, a magic item that's meant to be worn can fit a creature regardless of size or build. Many magic garments are made to be easily adjustable, or they magically adjust themselves to the wearer.

So generally speaking, magic pants should resize to fit a centaur, but there may be exceptions. Use Magic Device is unnecessary.

In fact, the description continues,

Rare exceptions exist. If the story suggests a good reason for an item to fit only creatures of a certain size or shape, you can rule that it doesn't adjust.

but notice that this is something special about the item—some good reason why a particular item wouldn’t adjust. Not something special about your body. The rules say that typical magic items adjust for any body size or shape.