tiny mobs only gets on fire if there are normal sized mobs on the same block

i was making a sheep farm. all my sheeps are standing on one block: a hopper. when there are tiny sheeps and a fire charge was placed only the tiny sheeps did get on fire.

i tested this out on other mobs that you can bread. and only rabbits did both get on fire. i only tested on friendly creatures.

sorry for bad english... i am dutch