What is an “undersized” squad?

Is it less power level or fewer characters?

I once LOST a Grand Arena match by 1 point, cause I killed a 5 person squad with a 4 person, cause I needed an “undersized” accomplishment... it was NOT worth it.... that 5th team member cost me the 5 points for full health plus 5 points for protection.... thus lost the arena match..... so what does “undersized” mean exactly?

Thanks,