dents vary of course but in steel frames most are not entirely susceptible to merely being filled (eg with car body filler); the reason for this is that most dents have raised regions nearby the dent. [if you want to see why, roll a piece of paper into a tube, and make a 'dent' in it; you will soon find that you cannot do this without making a raised area near the dent.]
This means that if the dent is merely filled until smooth, the tube diameter ends up slightly larger than previously. If the dent is filled with braze metal, the thermal cycle tends to shrink the surrounding metal and of course a framebuilder may knock any upstands down.
Large dents in thinner tubes (eg top tubes in racing frames) can usually be rolled out, to the point at which the surface is smooth enough such that it can be refinished with a couple of coats of filler-primer (and any increase is diameter is not noticeable), anyway.
cheers
