Any radio transmission occurs at a certain frequency. Depending on the propagation speed of radio waves through the space (or the air), which we consider a constant (approximately 300,000 km / second), a frequency corresponds to a wavelength:
Wavelength = Propagation Speed / Frequency
The geometry of the antenna is based on taking advantage of resonances, so their physical dimensions are closely related to the wavelength of the signals we want to transmit.
For practical purposes, this implies that an antenna generally will be bigger when it is designed for lower frequencies.
If we refer to the bands used in FPV, the approximate ratio of sizes for a particular type of antenna will be:
1.3 GHz → 1
2.4 GHz → 0.54
5.8 GHz → 0.22
In other words, a dipole antenna is approximately twice longer for 2.4 than to 5.8, and four times longer for 1.3 than for 5.8.