The difficulty of the map is actually based a lot on the number of aliens compared to the size of the map. Eight aliens on a small map is a lot harder than eight aliens on a large map. For that reason, just increasing map size would actually make the game easier.
However, you're right that medium-to-large maps offer more strategic possibilities. We'll always want a mix of small/medium/large maps, but we still need a way to balance alien numbers to map size. Most of the maps created in the last year are larger, but many of our maps are still quite small. We don't have enough mappers to make enough maps, so map adjusting will always be a work-in-progress.