I saw my hamster mating ...but that"s the last time....they didnt do it again...why?

my brother but me a siberian dwarf hamster...it"s gold and black....can they mate???or no because the girl is so thin while the boy is so big....and tthe other problem also is they are so small....different like i saw in the youtube.com videos their so huge...why??/please give me an answer please... :)...im 12 yrs. old..plkease give me an answer...please brother s and sisters