If we take a step back, rebels have a huge theoretical advantage when you look at things from a 'balance' point of view.
Think for one moment, if you spend the time stockpiling, you can get as many Rebels as you want, all with Radios, some form of High powered weapon with plenty of ammo (Semi or Fully automatic rifles/Shotguns) and over 100 armour EACH. There's no limit on how far you can extend, no limit to how long you stay outside. Just your own stockpiles. Vortigaunts help too with the Sniping department.
Combine can only keep 12 out at any given moment, including scanners and synths and 90% of them can only use MP5/7's. The only thing going for them is consistency, you can reliably get a handful of cops/ota and expect them to know what they're doing. They will have a weapon with plenty of ammo, they're all on the same radio and they will have full armour. Add the force modifier that the Combine have a clear 'Structure' to follow and tend to join a VC when they make a PT. A Rank Leader/Ordinal can just say "I need 5 People" and get them immediately, regardless of what that task is.
At the end of the day, Time to Kill is quick, number of men and structure really help in engagements, especially if they have experience (EOW). It's unfortunate I wasn't there to see what happened yesterday but I hope you had fun.