First thing would be, it leveled up.
Because it's level went up, and damage is proportional to level?
commented 47 minutes ago by sumwun
Yea that. When a Pokémon increases it's level, said Pokémon's damage output also increases.
Now, when it's level 24, which is the double of level 12, + it evolved, I guess it's evident that the damage output would be increasing.
A Zubat has base 45 Attack and 30 Special Attack, whereas a Golbat has Base 80 Attack and 65 Special Attack. The base stats itself show that the same attack used by a Golbat would have much more damage output than a Zubat using the same attack!
So now we come to the conclusion that, the increase of levels, plus the evolution, has aided the Zubat in becoming a ZuBat Man.
Hope it helped!