It's not anywhere close to impossible, to stretch the curve horizontally.

However, using the red curve as a point of reference isn't so simple, because the increased gradient of the green curve (which gives it the extra height) in turn has implications on the horizontal range, which are not as simple as just "longer" or "shorter".

If we were to look at these two curves on a flat surface they'd have almost the same horizontal range - a pixel or two different, due to that the projectile starts at (roughly) the lemming's head height, not at ground height. Above this, the green curve has more range, below this, the red curve does.

See the new attached image. It now has three white lines - the highest one is 16px above the projectile's starting height, the middle one is at the projectile's starting height, the bottom one is 16px below the projectile's starting height. Note that which one has more horizontal range, is different at each line - the green curve does on the top line, the red curve does on the bottom line, and they're equal on the middle one (which is what I actually used as a reference for the new curve - kept the same "at starting height" horizontal range, while making the peak higher).

I'm not the best at explaining this; if it's not clear, hopefully someone a bit more maths-y can do a better job of it.