Yes, it is a big deal, to me anyway. I am coming from an HTC Rezound. The extended battery for the Rezound made a big difference in the size / shape /weight of the phone, and I did not like it. The good news is, so far, the S3 with the OEM battery is doing better than the Rezound with the extended battery.
I really like how slim and light weight this phone is. I almost went with the Razr Maxx HD, just to have a bigger battery and still be (fairly) slim and light weight.
My wife had the T-Bolt and I had the big extended battery on there for her. That was big. I have the Hyperion 4200 on my S3 and it is much nicer and slimmer than the T-Bolt. If you want more battery capacity on your S3 you will have a bigger phone, that's inescapable. I am averaging 20 hours to get down to 20% with this battery.