I think i have a defective iphone4 memory is this possible?

My iphone has 264mb of music, 87.4 mb of subway surfer, 50.4 mb of hayday, and 36.5 mb of photos and my phone says that i have 129 mb left and 6.2 gb used somehow and the werid thing is that earlier it was different an less then more and i didnt do anything so is it posible the memory might be defective or did i have to do somthing plz help and my sis and friend hacve the same phone and have way more apps and stuff and are big mb somthing dont add up plz help and is there like a sd card that i can put in my phone