When I ride into my new bike the fork doesn't shrink. My weight is about 81kgs. I think good forks usually shrink to 75% of its length. In my case it does not shrink at all.
The fork is cheap. Does not have any preload settings with it. Also is does not have lockout functionality.
Is there any way to make it softer? Maybe by opening it and do some hacking. Not sure though.
The fork manufacturer is Zoom. Here is the details.
My fork is same. Except its color is white.
Fork specs clearly say peload and lockout despite your claim that it has neither. However, looking at the image, I only see preload knobs. Not sure where the mechanical lockout lever is... So I have two guesses either A)it's locked out and the lockout knob is not very obvious, or B)The fork is inexpensive and stiff and therefore you are not experiencing any "sag" (The compression to ~75% when the fork is loaded with your weight). The perception of lack of sag is likely exaggerated by the fact that the fork only has 50mm of travel. As you get into mid priced forks, most start at 80-100mm of travel.
lockout, so it is most probably locked out now. Identify the lockout and preload adjust knobs and make sure the lockout is disengaged.