ImagineMaker
chakra reserves are said to be influenced by the strength of the body and the strength of the mind...his body might not be that strong but his mind should be considering he is a reincarnated person. Uzumaki have a large chakra reserves because their bodies can naturally become strong on their on, they also have a talent for fuinjutsu, while also having the minds eye, adamantine chains, and an extremely high vitality that makes them both resilient and give a small regen. The senju on the other hand have and extremely high chakra reserve because they are more attuned to nature energy and as you know nature energy strengthens the body over time so they could continue to become stronger with time as they keep absorbing nature energy, Hashirama was said to be able to enter sage mode instantly and maintain it as easily as breathing ad such a high abundance of nature energy constantly going through his body gave him what amounts to super regeneration as he could basically regrow entire limbs mid battle, senju's are said to have an extremely long life span. comparable to that of the Uzumaki who's vitality is so strong some can actually survive having a tailed beast removed from their body/mindspace.