Actually it's 1/10 and 3/25, so in theory you have more ressing power in 25, but there are more people in the same space, more opportunities to be killed too so it probably evens out.
Yes, deaths are far more serious in 10 man. It could be argued (not that I am making that argument or not making it) that death is more likely on an individual level during 25 mans so it may even out.
And while it is true that 10 man is less people in the same space, ranged has a lot of breathing room now and most encounter areas are huge.