Minecraft wiki entry for Sculk_farming, associated text: As stated before, mobs only drop XP if killed by a player or tamed wolf, but if a mob dies near a sculk catalyst, regardless of how it dies, the catalyst will always produce sculk blocks, which can then be mined for XP.
Mobs will still drop their loot but not their XP. The amount the growth spreads depends on the amount of XP the mob is meant to drop.
When a mob that drops about the same XP as a zombie or a skeleton dies near enough to a sculk catalyst, the growth mainly focuses on a 3x3 area. This can be helpful for you to know how big you want your platform to be.