You're not wrong about there being a lack of certainty in this, but there's still a lot you can infer by looking at a person's size in relativity to the objects around that person. (That old "banana for scale" meme comes to mind.) Most bed mattresses are a a bit over 200 centimetres in length, and the MC looks like he takes up about 90-95% of a bed's overall length, so we can thus infer he's about 185 centimetres tall. And with that number in mind plus his overall muscular build, we can roughly intuit that he weighs somewhere between 90 and 100 kilograms. (Admittedly I'm skipping the explaining of a lot of technical mumbojumbo in this.)
"But what about the women?" You might ask. Well, we can pretty easily intuit their general level of weight from their height as well. Seeing as how just about every single lady in the game has been next to, on top of, under, or in some other way in close/parallel proximity to the MC, their height is easy to calculate as well. I mean... Roughly anyway! Alison is easily 15 centimetres shorter than the MC for instance, seeing as how she has to "reach up" to kiss him.
So! With all that in mind, judging by the aforementioned woman's overall body proportions, it's safe to assume she is of fairly average weight for an adult lady, which is about 60-65 kilograms. And using Alison as a sort of "standard" to measure the others by, you can further intuit their respective weights as well.
I mean... Yeah, I am just estimating stuff here, but I'm also the kind of loser that actually genuinely enjoys mathematics, so I'm pretty sure I'm at least in the right ballpark on this.
-Mal
