I tought I was pretty sure this is not the case, but some discussion has arisen and I'm not so sure anymore.
What I know:
- Most magical objects in the game auto-resize when worn by creatures of different sizes.
- There is a table detailing weight and cost of non-medium armor. This could still be useful for mundane armor, even if magic armor automatically resized
- Some spells explicitly tell us that armor does resize, as an exception to what happens normally. Again, this could be an exception for mundane armor only
- No distinction is made between a huge creature trying to wear a medium magic ring or a magic-ring-wearing human becoming huge by some polymorph spell. Might a similar distinction be in order for armors? (See my previous point)
I'll only accept the answer that quotes some manual (or FAQs, but only if no manual solves the problem) on the matter.
Best Answer
Armor and Weapons do not resize
Rules Compendium (p 84-85) says it more explicitly than the core rules do (emphasis mine):