Sparta is stronger than Athens because from the age of 7 they go into school not just learning about stuff for life but mostly about fighting and wrestling and things like that.
Athens may have more people than Sparta but there people aren't as tough and well trained as the people in Sparta. Also Sparta's constitution is better because it talks about obeying laws while Athens doesn't.
Sparta was also better than Athens because they had a better government and more freedom. In Athens women weren't even allowed to leave there houses, but in Sparta women were actually allowed to leave there homes.