Actually it's not a mystery.
- 350,000 concurrent users at PEAK (referenced many times).
- 250 servers (server status page).
- 1400 people per server AT MOST during PEAK.
- 2 factions, 700 per faction average (less on republic for sure).
- 20 planets including fleet.
= 35 average people per planet. Of course you will see higher numbers on fleet vs quesh but the average of 35 people per shard seems about right.
There are way too many servers, planets are way too huge, this game feels empty. It's also funny that most servers are listed as heavy which means the caps on them are very very small.