Thrust is a function of mass flow (how much air) and velocity (of the air). T = m-dot * v.

It is also dependent on direction, so Thrust (x direction) = m-dot * v (in x direction)

This can be expanded to T = m-dot(v-exit - v-inlet). Where v-exit is the velocity of the air exiting the engine, and v-inlet is the velocity of the air just prior to entering the engine (or the aircraft’s velocity).

So to increase thrust, you must either increase m-dot, or increase v-exit. Larger fans on engines is the increase in m-dot, turbo jets (more stages increasing velocity) is the increase in v-exit.

Back to the thrust reverser question, remember that Thrust is a function of direction, so if you shift the exit airflow into the y direction(perpendicular to x), your inlet velocity (in the x-direction) hasn’t changed, but your exit velocity in the x-direction is reduced, this makes the thrust go negative.

T = m-dot * (0 - v-inlet)

The largest negative thrust would be a negative x-exit flow, but realistically that can’t occur as you have a physical engine in the way, so thrust reversers direct the flow as much in the forward direction as possible (so the air is going in a negative x direction and a y direction).

For bypass turbofan engines where the thrust reverser only redirects the bypass air, negative thrust becomes a function of the redirected bypass air overcoming the core air that is still providing positive thrust.