A walkie-talkie requires a big antenna and consume a lot more power than a cellphone.

Both not true.

Both European PMR446 and the US FRS are limited to 0.5 W; GSM uses four times that. There are walkie-talkies with very small antennas too. The limiting factor is line-of-sight, in any case.

If you're fine with less than real-time audio, you can get much, much smaller and low power.