Mail order patterns were in nearly every publication directed to women, and the fast majority were available in much large sizes than the name brands. I mean even Butterick has roots in mail order. Sure, I have some Simplicity patterns in a 50 bust, but I have seen three times as many in mail order form, and one in a 56 bust!
Id like to think the reasoning is the same as Lane Bryant, mail order was a smaller demographic, but with it you could cater to an audience who didn't have much representation. A lot of store patterns start at 30 bust and stopped at 38 bust, or 48 bust in some of the more expanded lines. Ive seen a vast majority of mail orders start at 38 bust.
So the drawings might not be as pretty, and often the envelopes were whatever the company who ordered them was able to ship them in, and true, some of the styles were a few years behind the trends, but if you wanted options in size, price, and style, mail order for the stout woman was the way to go.
Here are some mail orders from my personal collection.
The above are 40 bust patterns.
The below shirtwaist pattern is a 42 bust and the other is 44 bust.
The above two are 44 bust and I also have the same underwear pattern in a 50 bust! The two 30s patterns below are 46 and 48 bust.
The below patterns are 48 and 50 bust.
Also with mail orders I often find some of the most comprehensive tid-bits of instructions on general sewing Ive ever seen. These were all on one pattern I had.
This is the most comprehensive size chart Ive ever come across.