Why do I weigh more the day after leg day?

On Ep. 104, Nikkiey compares it to a repair crew. Strength training breaks down muscle fibers, and your body sends in help with lots of water to rebuild them stronger. Along with that comes inflammation, which she describes as caution tape around the construction site. It makes you hold even more water until the repairs are done. She is clear that this is good inflammation, not the chronic kind linked to disease. On Ep. 139 she adds that the water sits inside the muscle, carrying in the nutrients it needs to recover, which is also why you might look or feel a little puffy.

Why leg day in particular? Nikkiey points out that legs and back are the biggest muscles in your body, so they demand the biggest repair job. On Ep. 245 she says the more sore you are, the more likely your weight will be temporarily higher. On Ep. 104 she says to expect a pound or two. On Ep. 135 she adds a bit of nuance: the day after a hard session, the scale could go up or it could go down. Either way, you did not gain two pounds of fat overnight.

If you just started lifting, she notes the scale may not budge for the first few weeks while your body adapts. That is not a sign the plan is failing. Keep training and keep it in context.