It's more complex than that. There are a number of other factors at play. Multicolored cards are stronger than single-colored cards. Certain creature types (angel, dragon, etc.) tend to get more powerful creatures for the same cost. Different colors get different strengths of creatures for the same cost (White has strong low-mana creatures, Red gets creatures with high power but low toughness more efficiently, Green just gets big creatures in general). It's also not a 1:1 relationship between mana and P/T; 4+ mana creatures scale up much faster than +1/+1 per mana. Also, recent sets have been moving to still more powerful creatures, so old balancing guidelines don't apply with the new power creep.