I have this problem with my Ollie who is now coming up to 15 years.
He has lost a lot of weight recently so took him to the vets and he had full bloods and tests done and is apparently fit and healthy.
As bizzylizzy said it is purely muscle mass loss. Ollie’s bottom is a lot smaller [wish mine was] 😂, generally skinnier all over. I did try feeding him more but it upset his tummy and just made him poo more, so just reduced it again and he is fine. It’s just old age really, so as long as there are no underlying problems, maybe just the odd extra treat or so.