Honostly, smallers maps are more difficult. On larger maps you can always go somewhere and taking a few more planets can be quite effective when finding some good bottleneck position. On smaller maps you are quickly breathing on some coreplanet, which in turn feels a need to breath back a bit .. with mk4 coffing & sneezing.
Also the FS-campaign gives you a lot more room to let it trigger the recoveries in a way they actually work in your favour. Doing it on smaller maps means you will be recovering later shards through/from core/home-planets.
And dont forget about the FS-fleet itself, without that the actual taking of the planets would be a serious grind(read: boring), defending would again be twice as interesting, but fail once and you likelly fail the game (its fairly difficult to stop 7k mk4 ships if you just wasted your own fleet
).
Some things you just gotta do once in this game though .. pretty much anything, at least once