Why Antenna loading means adding reactance, usually a coil, to make a physically short antenna behave electrically like a longer, resonant one. A quarter-wave vertical on 40 meters would be about 33 feet tall, so a mobile whip a few feet long is made resonant by inserting an inductor (a loading coil) at the base, middle or top. The coil cancels the capacitive reactance of the too-short element so the feedpoint looks resistive at the operating frequency.
Watch out A resistor would not tune anything out; it would just waste transmitter power as heat, and a base spring or beefed-up elements are mechanical fixes, not electrical loading.
Loading = coil, not mechanical. Short whip plus inductor equals electrically full size.