I think if you vacuum packed the fresh (they're not wet dammit) hops then they might compress as much as the dried ones do, but it's tough to measure that way. But volume is a reasonable way to do it for fresh hops if you have a little experience with it, I only do one fresh hop beer a year, my "Three Colander Pale Ale". I add a colander full at 60 min, another at 15 minutes, and another one at knock out. Te hops don't get get much fresher since I'm picking while I'm brewing. The beer gets consumed rather quickly and I've never entered it anywhere, but we like it. It varies in bitterness a bit from year to year, but that's part of the fun.