The rubber bumper on the upper A-arm is a stop that controls how much the suspension "drops" when you lift the car. When you have the engine out of the car, the drivers side expands until the rubber bumper stops the movement. The pressure of the spring makes it hard to flex this side up and down.

The passenger side spring can expand a bit further, since there no rubber bumper there. When the spring can expand a bit more, it doesn't feel as "hard" as on the drivers side. Therefore it is easier to flex the passenger side up and down.

Try fitting a rubber bumper to the passenger side before you take things further. I'm pretty sure it will solve your problem.