The Bladderwort Test

There seems to be a bit of a leap of assumptions here. The claim is that, since this organism is doing just fine without all that non-coding DNA, it must not, therefore, serve any purpose in that organism. The conclusion doesn't really seem justified.I've heard to argued that a lot of 'Junk DNA' may serve a function in the sense that it keeps particular parts of coding DNA distant from each other. The DNA of the bladderwort may have evolved a particular formation that meant most of this DNA could be safely removed without harm, but this doesn't mean you could do the same thing to another organism's DNA - since it may require a different formation to function effectively.By analogy, we could point to the fact that the tail has shrunk to a vestigial remnant of its former self in ape evolution. We would not be justified in concluding from this that tails are an irrelevance with no function in the animals that still possess them.

Seeing this thread pop up again reminded me of something I read the other week, in a discussion of genome size in birds. Birds, apparently, have smaller genomes on average than mammals, and I read posited somewhere that this was about weight reduction for flight. Whilst this idea was dismissed as silly, since the total weight of all an organism's DNA doesn't amount to much, it was pointed out in response that birds have smaller cells than mammals, and that cell size correlates very well with genome size.Is it this simple? Is the size of a genome just a matter of how much space there is in the cell? Are there mechanical reasons to do with DNA function why a bigger cell would need the coding regions more spread out?