The reason the seat tube will bulge at the lug shoreline is because it is not at the same temperature as the sleeve and expands more quickly. If it got hot enough it wouldn't return to its former shape. This can easily happen when spotting a frame together. It is easy to get the thin top of the seat tube red before the sleeve/lug is up to brazing temperature. The solution is to use a bigger flame held back further so it is heating a broader area and bringing them both up to the same temperature at the same time more slowly. A flame held too close to an exposed part of the seat tube can expand it faster than a heavier sleeve. Be careful with your flame pattern. It is common for those on the earlier part of the learning curve to be a bit sloppy with their flame pattern. The don't keep the point of the flame a consistent distance from the tube and perhaps get too close and hold it there too long on just the seat tube and suddenly it is red and bulging.