I would agree with fcma172 and cloankblade, except that you see her naked and she lacks that muscular definition. It even gets brought up later that when she isn't using her mana, she's actually not all that strong (mentioned during UBW date with Rin - trip to the batting cages).
Of course, this doesn't address how she would have been such a skilled fighter BEFORE she was a spirit, but hey, Nasu.
She had alot of mana (ALOT of mana) when she was King. That and how Excalibur is a magic sword. She probably wasn't bench-pressing her weight though.
However, I still say if she's properly excerted herself during battle, her muscles will be evident and clearly defined. In a hot-bath or laying in bed, she'd be more relaxed.
And while we're at it, let's stop making clones with different hairstyles. Oh, and let's give every character realistically-sized eyeballs and pupils. Also, the number of specular highlights on the eye should match the light source. Don't forget to stop the gravity-defying hairstyles and breasts.
Yeah, she even said she was too hard-bodied to be considered attractive as a woman. Not like Rin, who was lazy and fat.
Saber's body froze from her time that she pulled out the stone, so she could never add any more bulk than that of a lean 15 year old girl. For strength she would use mana bursts in her attacks. Rin is an crazy exercise buff, so the only one who would likely have any pudginess is sakura...
