The armor probably adjusts.
The magic items section of the Dungeon Master's Guide mentions:
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.
A GM can of course make an exception:
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 the basic assumption is that the magic armor will fit either form.
Bonus to Armor Classes
Some lycanthropes have different Armor Classes in their different forms:
Armor Class 10 In Humanoid Form, 11 In Bear And Hybrid Form
For NPCs this a separate way of calculating AC (natural armor), so it would not stack with the armor class calculated by armor since you can only use one type of AC calculation.
If you have multiple features that give you different ways to calculate your AC, you choose which one to use.
However, for PCs, the bonus to AC from lycanthropy would stack because it is worded as an isolated bonus in the "Player Characters as Lycanthropes" section:
and a +1 bonus to AC while in bear or hybrid form (from natural armor).