USSEP changes the burnt Astrid's appearance to match her standard appearance. I think the burnt Astrid having a lower weight than the healthy Astrid might have been intentional by the developers, as burnt corpses can often look pretty tiny. That is because obviously skin and muscle is burnt away, but also because the body quickly looses water. This results in them being a lot thinner and weighing way less than when they were alive.
This is the change in your patch notes:
"AstridEnd has been tweaked to better resemble her non-horribly-burnt self (eye color matched to Astrid, eyes, nose, lips matched to Astrid, weight matched to Astrid)."