An extreme example:
A year ago I went elk hunting with a Native friend, his brother and nephew. Friend and I are decently active, 180-ish when we should be 160 pounds. Brother and nephew were BIG – 300 and 350 pounds but active (plasterers). I got a 90-pound front quarter up out of the canyon and came back a lot faster than they expected. So I did it again. But those 140-pound back quarters? I couldn't do. Just could not. But those big guys? – they each grabbed one and had no problem, I took an 80-pound pack of small cuts and my clever friend carried out just the 20-pound antlers. We still give him crap about that!
My point (and I've heard this from PTs and MDs) is that inside of a big ACTIVE person, is a strong person. I didn't say healthy, neccesarily, but strong. If I went shopping, doing laundry, mowing the lawn, etc, all with AN EXTRA PERSON on my back, I'd be buff. Alas, if it is fat, you don't get to lay that weight down for your trip.
So I'll grant the "pounds are pounds" crowd that if you lost 10 pounds pack weight that would be very similar to having lost 10 pounds of belly last year.
But if the 10 pounds were liposuctioned off at the trailhead? Then it would be like you'd been in training with an extra 10 pounds on at ALL times – around town, in the house, etc.
So a hem and haw estimate – 10 pounds off your belly is like 5-6 pounds off your back. They both help, but you automatically condition somewhat for the belly weight.
But I find it helpful to think of dropping belly weight like buying SUL gear or a super-light bike. Skip the nightly beer and have one bite less of pasta at dinner for 3 months and 90×200/4,000 = 4.5 pounds lost. That's like 3 pounds off my back which would be very spendy AND I'm healthier AND I look better.