Windows 7 lag spikes - related to map size?

I've recently come to the conclusion that whenever I play on Windows 7, I only have these lag spikes in large size maps. Lower Ludi maps, some Leafre maps, those El Nath maps with the Dark Y&P and the Werewolves, etc. but not in, say, Ariant and Magatia maps, which are short in height and not really all that long, either. Nor do I have problems in Papulatus' boss room.

Is it possible that these spikes are related to map size?