"surely we would expect higher organisms to have longer or larger genomic volume."

You might, mightn't you? And in some cases they do, but often they don't and I'm not an expert in this area but Steve might be able to explain why is, or where research int his area is up to.

Your term "higher organisms" indicates an outdated and outmoded way of thinking. When taxonomists talk of "higher taxa" they are not talking about the complexity of the genome or the superiority of the organism but it's derived status and therefore it's assignment to a clade based on it's autapomorphies. It does not mean they are somehow 'better' than anything else (apologies if this is what you meant).