Slitherine

Why give the big kingdoms thousands of Legacy to start with?
That is really bad for multiplayer.

Byzant wont have much luck with its starting unless it fulfils all its objectives, ie. all that Belisarius and others reconquered.
It mostly prevents you from winning in 3 turns if all start at zero. So you have 3+ that have a "lot" of legit to prevent early wins.

It's part of the package, the benefits, when you choose a nation. Kingdoms does not try to balance nations; you'll have much less chance to win as the Isle of Mann compared to Castilla. There are special conditions, like the Byzantines, that need to fulfill missions as well.