Honestly, it doesn't require strength. I'm super skinny and barre chords became fairly effortless once I figured out how to play them right (it was quick fix once I knew what to do). Contrary to popular belief I don't use my thumb as an opposing force. The thumb and finger muscles are weak - they weren't designed to apply prolonged pressure.
Use gravity and your bicep. Pull back on the neck. I find it useful to think of the actions of the left hand being separated into two actions.
1. The finger muscles that allow you to get your fingers into the correct shape.
2. Gravity, with assistance from the bicep, pull the fingers into the fretboard.
I kid you not when I say a full barre, for me, requires no more effort or pressure than an open G chord.
If your applying pressure using thumb and finger muscles then your just locking in excess tension. You want those muscles to be used only for creating he right chord shapes - beyond that you are just creating more tension than you need.
When it comes to gravity,
here is a diagram I threw together ages ago to help show the physics. When you relax the shoulder (red), then the upper arm swings back slightly, which in turn pulls the hand toward the fretboard. When gravity isn't sufficient add a touch of force by pulling back with the bicep, but I have to stress, you don't need much force at all.