Answer:
Generally they can jump about as high as their back is tall. In 1998, the highest recorded jump in the Guiness Book of World Records was 8 ft. 1 1/4 in. Check it out here http://www.ultimatehorsesite.com/info/worldrecord_jump.html
---answer--- my pony jumped 155cm at a chase me charlie !!! he is 13.1hh connemarra and jumps 1m classes
Another reason (if it's on steroids) is that the hind legs push with enough force to flatulate under themselves. Some scienctists believe that this may be a key reason and is another reason why athlete can jump so high!