SP slightly change depends on your party size, this is done so that it encourages partying.

Comparative effort is not exactly true here, it doesn't matter if a party or one person kill a mob, it is still after all the same mob. A party of r20 killing a r30 Beastmen, will be easier compared to a r50 solo a r30 Beastmen? Or is it the other way? Between a weak Coblyn and a mighty Drake though, there is a big leap in term of SP/XP rewarded, because effort in killing a Coblyn (no matter who do it) is not the same as killing a Drake.