In fairness, that was the One True Ring of Power, and Sauron was the kind of guy who worked through more than a few bodies (if I recall correctly - this may of course be wrong), so he would want to design something with a "one size fits all" function. It was his life's work, his ultimate bid for world domination; shape changing ability, in this case, is common sense and probably not too difficult to build in as an extra. Sauron has to plan for the long term.

In D&D, though, every magical accessory has it built in... it seems to be part of the enchanting process (except for weapons). It's justifiable, certainly, since the item is magical already and many of them will have been designed for sale (and it's probably easier to construct one chape-changing bracelet than keep a variety of different sizes in stock). It seems to me, though, that it was put there mostly for OOC game balance reasons.

That said, I'm still happy that I can grab those gloves back. :)