The level and strength doesnt correlate.
Level only represents experience so a person can be level 90 and weak.
Only stats represent strength.
Mc gain strength from level up due to system assigning stats for each level up.
Mc gain a lot of exp from the first kill of any species and half stat for any other of the same species.
Example: mc kills human > gain 1000. Mc kills another human> gains 500 exp