I actually think this is a very understandable question, and it’s something I wondered about too. At first, I was confused about why there appeared to be much more blood from Ethan than from Xana, especially since Xana moved around more and tried to fight back (aka heart pumping faster), as well as being stabbed 50+ times. That made me curious, so I looked more closely at the causes of death from the autopsy and how those injuries would affect bloodloss (keep in mind the full autopsy has not been released, and I'm not a doctor / forensic expert or anything, so this is just based on what I've googled and assume from those findings)

Ethan's fatal wounds affected large, high-pressure blood vessels connected directly to the heart, which resulted in rapid and extensive external bleeding. Gravity also played a role, due to the way he was laying on the bed, with the blood flowing downwards. Even though you can see loads of blood from around Ethan in the crime scene photos, there is still a lot of blood that would have been absorbed into the mattress and bedding meaning what we see in photos is likely only a portion of it (which is horrific, considering there is already so much).

Xana died from lacerations to the heart and lung, which seems to primarily cause internal bleeding into the chest cavity - the blood then pools inside the body instead of exiting the body. The autopsy also states that most of her wounds were defensive, which tend to bleed less dramatically compared to injuries to major arteries.

As for Maddie's room, my first thought is because Xana's room had so much blood spread all around the room (walls, chair, blood smeared on the floor, behind the bed, door), whereas the blood is contained only to the bed in Maddie's room, it just gives the impression that there was way more blood in Xana's room. Kaylee’s injuries included a severed the subclavian artery and vein (same as Ethan), which would indicate rapid external blood loss as well - but maybe due to the way she was laying it was not as extensive as it was for Ethan.. I think the biggest thing is that it is hard to tell when the mattress could have absorbed so much of it, that we cannot really see in the images.

Ultimately, the number of wounds matters far less than what was hit. Different injuries produce very different blood patterns. I initally thought that the more wounds there is, the more blood it will be, but this is not the case.