Security needs more bodies in high pop, period. The current maximum sec numbers are best for around 80 pop. Anything more is too much.
Some antags spawn based on how many people are on station. Not how many security. There are sometimes traitor rounds that have 20ish traitors. When security is fully staffed they have a total of 12 bodies. One is a doctor. Two are supposed to stay in the brig. Three arent supposed to be directly handling antags unless in defense. Its honestly not even a good idea for the HOS to go in the field unless its an extreme emergency but they usually go supercop anyways and it doesn't help much.
Traitors are more well armed then security, especially if they work together cuz they can get gear that compliments each other (one with carp one with guns) while sec is stuck using the same equipment as each other, half of which is nullified once a traitor kills one newbie officer and gets a HUD and headset. God forbid if a freaking cling or vamp get thier hands on syndicate gear. Forget it.
Antags are also more likely to be given useful equipment by collabs or meta buddies whereas sec rarely gets anything extremely useful from science.
Antags are also are regularly tapped into sec comms and have the element of surprise initially whereas sec stands out like a sore thumb. Add in greytide who constantly harass security or cause other problems sec needs to deal with because they are bored that they didn't roll antag and hate authority.
It's also far more likely to get 2 or 3 experienced antags who have literally worked together countless times than it is to get a sec team that works together well (same antags love bitching about cliques...lel).
Also the general attitude of the community that authority sucks and bad guys are cool because half our players are teenagers who already have severe issues with the authority figures in thier lives (parents, teachers, boss) and want to vent thier frustration here because its a game and they are too young and dumb or just don't give a shit that sec players are also players as well so they treat them like crap and make thier jobs difficult as much as possible.