Do i have to defragment my omnia?

-i own an omnia i910 and it will give tell me to go to file explorer and clear some memory space and i will find files taking large amounts of memory that im sure i didnt download and also have no use at all ... today i remembered a similar thing happens to some computers and the solution is to defragment it ... could files be fragmenting on my phone as they do on my computer? ... if it helps my omnia is a smart phone and reffered to as a "pocket pc"