I think there's something that most of these answers are missing, but, unless I'm wrong (in which case please correct me), aren't the 5x5, 10x10, 15x15, etc. not the size of the creature, but how much space it controls?
I mean, no human is 5ft wide, and I'm pretty sure this has been explained as them controlling that 5x5 square. I.E. Nothing can stay within those 5 feet unless you let them.
Now you may think that I'm suggesting that a 'Large' or such creature must be smaller then the grid it controls, and in some cases I feel that's true, but let's look at dragons for a second:
A fair chunk of their body, at least in terms of length, is a long neck, tail, and wings. If we're fighting a 'Gargantuan' red dragon, how much of the area around it can it actually stop me, a lowly human, from standing in? Not near as much as it can reach.
Unless being directly attacked by a tail or bite, it's probably fairly easy for me to stand under its arching neck, or duck a tail. Similarly, I'd have to get up to its core body to really be able to hit at it, bar maybe a meaningless poke at a wing high above me.
For me, the base of this 30x30 dragon is just the space near its core that I can't maneuver around in. I expect the dragon could be twice, or even thrice, as large as that base, at least in length or wingspan.
Might vary quite a bit too, depending on age (since 'Ancient' or 'Adult' covers hundreds of years), gender, and/or subspecies.
The space occupied on the grid isn't how large a creature is, but how much area it thoroughly controls. A creature's actual size may vary considerably from that.