I have two early 1970's raleighs with the pre-oversized stays that could be considered as failed. The first is a 1974 international. The cap was inset to the top of the seat stay. The border between the stay and the inset cap was cracked, probably due to stress, so there is a line in the paint but that is all. The line is about 130 degrees around the stay cap.
The second bike is a 1970 super course. The damage has progressed a little further than on the international. There is a crack between the inset cap and the seatstay, for maybe 180 degrees, and moreover, the stay began to crack at 9 o'clock (right-hand-side stay), and the crack ran over to the vent hole and stopped. End of story.
So I can imagine that the oversized caps are probably easier to braze and perhaps give more surface area for brazing material (since the material can provide a fillet on both sides of the backside of the oversized cap, and more importantly, you don't have to file away half the fillet to finish the stay.)
- Don Gillies
San Diego, CA, USA