They're typically meant for older children who've outgrown diapers and need something they can pull on/off themselves but the x-small *might* fit depending on her size. Maybe contact ME with her measurements and see what they think.
As for absorbancy, the two diapers you're using now are quite a bit more absorbant in total than are the bedwetter pants. They're listed as having 21oz of absorbancy while the total of two One-Sizes and two liners is a total of 40oz.
Are you sure you need that much absorbancy in there? I mean, 40oz is probably more than you really need (she probably doesn't even consume that much liquid in a whole day) in order to prevent leaks. You could probably get by with just one diaper and two liners or two diapers with no liners maybe? Obviously the diaper would be quite a bit wetter in the morning but chances are that it still won't leak. For heavy wetters you either have to deal with a really wet diaper in the morning or a big diaper butt at night! Been there, done that so I know the feeling. I've taken pictures of my girls big nighttime butts because it was so funny to see!! ;-)
Remember too that the bulk might bother you more than it bothers her. It's surely a little ackward for her but she might not really even care and it's not like she's really moving around that much, she's just going to bed anyway.
Another thought would be to get a couple bamboo diapers and/or liners to use at night. They supply the same absorbancy but are quite a bit trimmer. You could consider getting the Lg Sandys or Toddle-Ease diapers, since they're bigger than the One-Size they probably spread at least a little of that absorbancy a little better (more on the hips and waist probably) so it wouldn't be as concentrated between the legs quite as much. Or maybe a hemp diaper or liners? They're also quite a bit more absorbant than cotton...