You win if we win

I've no idea, if you (or anyone else who plays mabi and reads this) could shed some light on the reasoning for leveling pets that'd be pretty cool for me

Pets skills and stats increase with level up.
The potential growth is entirely based on the individual type of pet, and the age of the pet.
Each pet gains a different stat bonus based on it's age when it levels, until they're older than 5 and stop gaining stats regardless of level.
All pets continue to ranks kills as they level, regardless of age.
Restful wind never gets more powerful, it's capped at level F on birth.
Nimbuses don't gain stats.

Compare -

Nimbus

Cosmic_Stallion

Spend a few hours roaming the Wiki. You've probably missed a lot of the basic mechanics people take for granted and exploit.