Hi there. I’ll quote something I found on reddit and which fixed the shrinking issue for me.
“I find this happens (if your inventory is 10 slots wide) only if your inventory is on the right. You can stop this behavior if you offset it from the right by a certain amount (too much for my liking). It’s some sort of invisible constraint that requires trial and error with your inventory positioning. I have my inventory directly on the left edge of your screen without issue. " http://www.reddit.com/r/Guildwars2/comments/2a91th/the_god_awful_inventory_shrinking_bug/cisvl04
Ever since I moved my inventory to the left side of the screen, the issue is gone (11 slots wide inventory).
I play on Windows , max settings most of the time, windowed mode on a 1920×1080 screen, with hidden bags.