It's not about the capacity.
It's about the properties of electromagnetic signal to propagate.
The longer the wave the harder to interfere with it. This is basic info even before highschool (under15yo). The bigger object have to be to do harm to signal that is propagated.
Longer waves can travel longer distances. That's why naval communication, or the off-shore mobiles (up to 120km from shore).
As you should know, the longer the wave, the lower the frequency. That means, the higher the freq the shorter the wave, the easier for smaller object to deteriorate it's quality.
The femto/pico/micro cells are deployed because
- It's harder for higher freq to penetrate inside of the buildings. Not anymore when you have your own radio cell at home.
- It's easier for RAN Engs to design, as in the hard terrain they can offer people in-home coverage.
- For islands where there is no coverage, but with good internet, such cells can be used to provide voice services.