The Scouting Legion or "SL" for short is a starter branch such as the Stationary Guard. The Scouting Legion is known for getting work done, where it is either being the most skilled in eliminating titans, or the fact that they gain a bad reputation for the large amount of casualties. May it be in a tie with the Stationary Guard for most easiest to get in, it is the hardest branch to keep up with. Activity demands along with the massive amount of titans being subjected to elimination in single missions are difficult. There are additional skill requirements if you ever want to advance up in the Survey Corps.
The Scouting Legion is responsible for fighting outside of districts, outside of the walls, or specialize in forests and in other sets. There is a special occasion, ever since the breach of Shiganshina of wall Maria, the Scouting Legion will operate within districts of active titan assault, or the districts of where the titans currently are trying to break through, AKA the first line of defense. The Scouting Legion is the only branch to fully use all flare-colors.
The Military Police has the most of all three branches. Due to it being a starting branch, it is relatively easy to get in, and this would explain it's member count.