Your last ladder game was on Regor, where you immediately mass stalled min 2 because you failed to properly handle a 4 mex hydro build order. Your 500 rated opponent did not mass stall. He has gotten double the mass you have by min 5. If this was a 20x20, you would have likewise lost by min 2 just as you did here. If this was a 5x5 you would have likewise lost by min 2. The fact it is a smaller map means you do not have the natural 5-10 minute lag to realize you lost the game at this point that would exist on a 20x20.
Part of the reason you get a lot of 5x5s at this rating level is that you need to play this smaller maps to get a solid basis of underlying hydro rush/no hydro BOs under your belt to actually have any decent games on maps like regor. It isn't fun to auto lose at min 2 only to realize you did at min 12.
Your last 20 games:
9 5x5
10 10x10
1 20x20
This is a totally adequate ratio to me and I don't understand why you feel you get "tiny map after tiny map"