My two notebooks prior to my 15" MacBook Pro (the original 2006 model,) were:
Sony PictureBook, a device that would pass for a "Netbook" today. It was low-end even when new, and I ended up running Windows XP on it at the end, and even with its ultra-pokey Pentium MMX 266 and 64 MB RAM, it was usable as my primary machine. (Which is why I find it funny when people complain how slow a netbook with a 1.2 GHz CPU and 512 MB RAM is.) I sold it four years after buying it for 50% what I paid for it. Incredible resale value on this puppy. I sold it to help buy....
PowerBook G4, 12" Aluminum. It was the minimal config, bought when it was the current model, but late in its life, and bought as a refurb; later upgraded to 640 MB RAM. My daughter still uses it to play games.
Then to the original
MacBook Pro, fully loaded (minus the $300 upgrade to add 8% CPU speed.)
So I had been using *VERY* portable machines for quite a few years before getting my 15"er. I had to go get a new laptop case, since my old one (which I loved dearly; the absolute *PERFECT* size for the 12" PB,) was just too small.
Moving from 12" to 15" alone, including bag switch, wasn't too big a deal. I have found that I use the shoulder strap a lot more, whereas on my 12" bag, I tended to just hand-carry. The big detriment to easy portability was the fact that with the bigger bag I started packing more stuff in with the laptop. My 12" bag could carry the laptop, power cord, plus a couple pens, business cards, small notepad, USB drive, and that was about it. My 15" bags (I've been through a few, trying to find one I really like,) tend to get packed with a lot more stuff ,weighing it down.
If you can keep your accessories to a minimum, a 15" shouldn't be a problem at all.