I haven't studied guitar strings, but harp strings are generally tensioned to approx. 70% - 80% of their tensile strength. To raise the pitch of a string one octave, the tension must be doubled (if I'm recalling the equations correctly) assuming everything else is kept the same. So raising 1/2 step increases tension by about 1/12, or a bit over 8%. So you start at 75% tensile strength, raise the pitch 2 half-steps - now you're at 91% strength just sitting there. Now pluck the string nice and firm... *snap*
I suspect similar for guitar. Going to a heavier guage means you'll have to crank the tension up even farther to get to the pitch you want - and the string will still be that much closer to breaking. Likewise going to a lighter guage - the tension will be less for the same note, but the string is weaker and so still close to breaking.
Only effective way to change pitch significantly while keeping the length the same (i.e. not using a capo) is to change the overall density and/or diameter of a string - different materials, wound strings, etc.
As others have said, use caution about increasing the tension on an instrument significantly beyond design spec. Guitars may not be quite as near to breaking as are harps when in tune, but they can still be torn apart.