Pokémon - Image #641,432

A bit of explanation.
Ash's Charizard is larger than most by canon. The 5'7 statistic is just often the average height of the Pokemon (this could also explain why it has more strength than the average Charizard). Take a look at the Super Smash Brothers Brawl trainer for example. He is based on the 16 year old Red from the Pokemon Adventures Manga. Assuming he is Japanese, he would be roughly 5'4-5'8 in height (assuming he is nearly done with puberty and the average Japanese male is 5'7 in height). His Charizard is about 3 inches taller than him making it about 5'10-6'1 in height. Assuming that training a Charizard for fighting increases the pokemon's height, then it would be perfectly acceptable for the average un-trained Charizard in the wild to be shorter than we would expect.

Sources:
http://bulbapedia.bulbagarden.net/wiki/Charizard_(Pok%C3%A9mon)

http://3.bp.blogspot.com/-IOpPmYyNu5c/Tz5-mWKW29I/AAAAAAAAL38/NyqcaMOPM0E/s400/SSBB_Rock_Smash+Charizard+pokemon+trainer+nintendo+wii.png

http://supersmashbros.wikia.com/wiki/Pok%C3%A9mon_Trainer

http://bulbapedia.bulbagarden.net/wiki/Red_(Adventures)

http://en.wikipedia.org/wiki/Human_height#Height_and_health