1) Servers in "suggested" area are those where my friends are playing, or some (random?) ones with around 20 players
2) Servers are listed from the most "populated" to the least, and by players rankings range (for example, servers where players have all a very high rank (around a million let's say) will be after the ones with players with rank similar to me)
I see when not-very-popular servers, there are much players (like 25 in a server where usually there are 5/6, which would be pretty much your situation) it's because it became a server with "Red arrow" for you, and green box for newcomers, meaning it has only players that have a very few LP. In this situation, since servers with newcomers only are way less, more will join it.
This is from my personal experience