Most receivers of rifles for .30-'06 are long enough that you can seat the bullets pretty far out. This lets you get more push for 180-grains on up to 220, compared to the .308.
Years back, I read that factory loads for the .308 ran 53,000 psi, but for the '06, 47,000 to 49,000. On account of all the older rifles in '06. So, loading a modern-made '06 to 53,000 gives a couple of hundred ft/sec more punch, throughout the spectrum of bullet weights.
The Federal High Energy loads in a 26" barrel give 3,150 ft/sec to a 165-grain, according to a guy in Australia who's chronographed them. This agrees with what Mr. Federal sez on the box.
I've always joked that my Wby with its 26" barrel is a "cheater". That is, guys with 22" .308s can't get the MV I do with my handloads, and a .300 WinMag with a 24" isn't that far ahead of me.
About every ten years or so, somebody writing for the American Rifleman does the proverbial test of chopping one inch at a time off a barrel. Sometimes, two or three different cartridges for the testing. Off and on since 1940, then, I've read that for cartridges like the .308, it's around 50 ft/sec/inch. For the '06, around 75 ft/sec/inch. For 7mmMag or .300 WinMag, around 100 ft/sec/inch.
SFAIK, the "why" has to do with relative case efficiency. The '06 is at the edge of "overbore", and the maggies are moreso.
The .308 was among the first to be computer-aided in its design. It works pretty good in a 22" tube. That's why you see me recommend it over the '06 for those not wanting a 24" or 26" barrel for their rifle. A short '06 is gonna have more muzzle flash and the longer action = more weight, but no more MV than the .308.
And now you know as much as I do.
Art
PS: I've always liked the doggerel I once saw in a Cooper Column:
"Ain't many things a feller can't fix,
With a few hundred dollars and a thutty-ought-six."