A short bloke will have to take more paces in a eg 10 mile bash than a tall bloke. But he may weigh 10 stone while the big bloke may be 13 stone so in effect is carrying less weight.Whilst not infantry and a while a go since I served, My corps was very much mixed, both officers and ORs The best of the women matched up to the best of the men in terms of fitness, skills and general military allroundness. The worst of each match the worst.
But the size to power ratio did make it easier in terms of equipment carried for the average man, however as has been pointed out the Army contains many men of smaller stature.
Compare a 5'4" bloke with a 5'4" woman and the significant difference is the musculo skeletal system.
I have the highest regard and admiration for women who pass these kind of courses but am pretty certain it will impact their bodies in later years more than the male equivalent I mentioned above.