According to the university of Wisconsin, which is where about half my stock originated, the BBW and a show Royal Palm were the breeding stock, but it took a few generations for the desired genes to stick. I've read on some websites that it was the Holland, but since I am not Smyth, nor was I involved in the original breeding program I couldn't say. Since the UW were in ownership of the stock until they sent it into small farmers hands I tend to believe them. I also see the personality traits of the BBWs condensed into those little twerps. They do have a powerful personality packed in their little bodies. I call them my lap turkeys as they love to sit and get cuddled.
I've also seen size suggestions of 8 to 12 for a hen and 15 to 25 for a Tom. I think if you want smaller than you should breed or buy stock from smaller, or visa versa. I purchased stock from three different strains. Hence the size difference from 10 to 20 lbs in similar ages birds. I heard from someone that had a 7 lb Tom for dinner. It's all in the breeding. We have a small Tom and a large Tom and two young jakes ready and willing to step in along with 7 different sized hens. I don't raise them for show and could care less what would be recommended for that. I breed for the purpose of selling to customers that want a specific size. To me these are the perfect sized backyard turkeys. They are super personable, they are extremely hardy, they forage for most of their food even through the winter, they are healthy and seem the be immune, as predicted in many websites, to many diseases. Although I've never had a sick turkey I'm still cautious.
As for the Midgets use in creating other breeds, I couldn't say for sure. I do think that the midgets and beltsville were created separate from each other. Also from my limited knowledge of Hollands I think they go way back. I think they were a recognized breed around 1875 or so. Both the Midget and the Beltsville Whites were a recent addition in turkey types. The Beltsville whites being developed around the forties and the Midgets the sixties I think. Yes they are shown in the same class, as the Midgets are not yet recognized by the APA. Whether they ever will be doesn't matter to me one way or the other. As I previously stated I don't care to show. I'm not all about standards. I have much more interest in raising a healthy flock that suits my purposes. I breed for need.
Also since the beltsville whites were developed around the forties and the midgets not until the sixties if I remember correctly, I would assume since they were originally competitors for the turkey market, that one wasn't used to breed the other. I have no idea what was used to create the beltsville whites but they are also on the endangered list. The one thing I do know about them is that their breasts are not as large as the midgets. Beyond that I think they are cute little buggers too! I wonder if they have that same type of wonderful personality.