Could be that mediums are too much for that guitar. Take a ruler and lay it perpendicular to the fret board so that it passes the sound hole and the end sticks past the bridge. If there is a depression or trough under the ruler and a bulge behind the bridge then you have a problem with the bracing or bridge plate and the medium strings are too much for the guitar. If this is the case then it will look like the bridge is tilted forward towards the soundhole. Also I would get a dentists mirror and check and make sure that the braces are not loose or coming unglued. The bulging can affect the intonation and action on the guitar and if a brace is loose then it could ultimately be a warning sign of impending implosion, but I have seen a bunch of older guitars that have lived with bulges (older guitarists too
). It ultimately is something you should get fixed by a luthier unless you are supremely confident in your woodworking abilities.
Also, over humidity can cause bulging. Silica packs and some other products can remove the humidity over time if you keep the guitar in the case with them, but be warned you can go too far the other way too.
The bridge doctor can correct the bulge, but who knows what will happen to the tone of the guitar. My brother used one on his guitar and it worked. I cant speak for the tone because he plays so softly that its hard to tell if it is affected.
Goodluck!
Rich