How come my dog is fat but only gets fed once a day??

My er spaniel Ollie only gets one meal every evening/afternoon however he does occasionally get left overs from my softy of a mother lol and rather a lot at that, he gets one or 2 long runs a day normally, unless I'm just mistaken and he's just fluffy XD but I dunno he looks quite chubby to me, what do you guys think, here's a pic of him a href="http://www.flickr.com/photos/24509144@N02/?saved=1" rel="nofollow"http://www.flickr.com/photos/24509144@N0…/a