Here is the full list of all 14 Side by Size achievements.
Complete levels 11-21
Complete levels 1-10
Complete levels 22-30
Complete levels 31-41
Complete levels 42-60
Complete levels 61-72
Complete levels 73-81
Complete levels 82-98
Complete levels 99-100
Complete levels 101-120
Beat the game
Found a secret level
Complete all secret levels
Complete ALL levels in the game