Speaking in very simple terms, reducing the length of an antenna decreases capacitance, so to make it resonate at the same frequency as a dipole of 1/4 wave monopole you have to add inductance. A helical antenna is basically a coil, get the dimensions of that coil and number of turns right and you can add just the right amount of inductance to resonate at the same frequency as a dipole or 1/4 wave monopole. However shortening the antenna also reduces its radiation. You'll see some compromises that have bottom coils and the rest is straight, but the result is the same, the coils (and the helix) add inductance.
Mark.