Radio guts, hardware cryptography modules, power amplifiers, and batteries. They've shrunk over time but still aren't small. When I first joined the Marine Corps, we had ground radios for each of the major spectrums: HF, VHF, and UHF (LOS and SATCOM). Now, most of the radios are multi-spectrum (combined): HF/VHF and VHF/UHF/L.

The AN/PRC-117G is an example manpack radio used by the Marine Corps today. We use it for voice and data.

https://www.harris.com/solution/harris-falcon-iii-anprc-117g...

If you're not familiar with radio wave propagation, don't get suckered into thinking hand-held radios can always replace manpack radios or that manpack radios can always replace vehicle radios. The art of radio involves a balance between power, proper antenna use, and terrain. Small, hand-held radios often don't have enough power to transmit a signal the distance you may need it to go in whatever terrain the person is in.