Why is Yukio more muscular and well-endowed than Rin, aren't they supposed to be twins?
Twins, but not identical (only one has the moles and needs glasses, for instance), but also Yukio's been training and "working" much longer than slacker Rin. I expect his body is trained to increase his performance and survival chances, and that takes a dedication and persistence Rin lacks the devotion for.
Yup
As much as I love this manga and believe me I do, I already read this part. Re-uploads are really annoying!>:(