The bug
When a mob converts into another mob, its previous attributes aren't retained.

How to reproduce
Summon a mob with some attributes and convert it (copy the command below to summon a zombie scaled up, it will convert into a drowned).

/summon zombie ~ ~ ~ {DrownedConversionTime:100,Attributes:[{Name:generic.scale,Base:6}]}

See if it retains its scale after conversion.