Several years ago I installed Caravelle brass grip frames for customers. These were "premium" frames that could be fitted and finished in an hour. Unfortunately, Caravelle is no longer in business. Maybe I expected this brand to look and fit as well as the Caravelle.
This frame is a MK Technologies that I ordered from Brownell's ($99.95 retail, $89.95 wholesale). From the write-up in the catalog, I expected it to be a little over sized but this one was WAY over sized. I also expected the finish to be better. Instead, there were casting marks, discoloring, and some pock marks on the back strap and trigger guard. I was able to dress them all down so maybe it was good to have it over sized. I used a vertical bench belt sander with a very fine grit to fit the sides and ears. After I got it fitted, I used 400 grit sandpaper to dress down the fine scratches, then buffed it with 550 grit compound on a muslin wheel. This took me about 4 hours to complete. I must say, it turned out way better than I expected, considering what the grip frame looked like out of the package. The brass grip frame makes a huge difference in weight and balance.
One significant problem I encountered was the hole drilled for the grip locator pin was at least 1/8" low and 1/8" to the rear. When I installed the grips, there was a 1/8" gap between the top and front right angle area of the grips and frame. I had to drill new pilot holes in the grips to make them fit.
Your observation about the backstrap (actually the entire grip-to-frame fit) being too big is quite accurate. I did not alter the factory grips but I could have trimmed the grip frame down more. I actually wanted the brass to show from the sides. I did contour the edges so it feels great in my hand. My issue was, what good is a brass frame if you can't see it?