Q:
Does size matter?
Yes and no. The rewards a chest gives is hardcoded into them, so it’s possible to get decent loot from a small chest that was a reward for completing a Group Event DE (like the one at the end of the Champions of the Sun meta-event in Caledon Forest), while getting crummy loot from a big one (at the end of some Jumping Puzzles in the same zone).
What you should generally look for is the name of the chest. Chests that are just named “Chest” usually offer piddling loot; they are intended more as little bonuses while running through an area. Chests with more descriptive names like “Splendid Chest” or “Glorious Chest” are usually found at the end of Group Events and give better loot.