updating apps on iphone?

[shopping_for_pies]

i have loads of apps that need updating, but about 3 or 4 of them are over 20mb and i dont know why, but i cant update them even when im connected to the computer and the internet and itunes. is there a way of updating them on the computer from itunes? and if there isnt, if i click 'buy' on the app that is over 20mb that needs updating, will i be charged, or will it just update the already existing one thats one my iphone?
thanks!