I am a big guy at the moment(17 stone) and a few months ago was wearing my good pair of trousers that cost £45 and the seam(I think its called that) the line in the middle of the trousers at the top had the tread come out so I got it sewn up at a shop, a month later it happened again so got it done elsewhere.
I got a new pair of trousers 2 weeks ago and the same thing has happened! these are all brand name trosuers, the £45 ones were Next and the £20 ones were BHS.
I assume its because I am fat? the waist sizes are fine though and my 2 cheap pairs of trousers havent had a problem(though are getting tatty so dont want to wear them)