What is the reason the output signal I obtained from full-wave bridge rectifier has changing amplitude?
Take a careful look at first two oscilloscope photos. Your 'scope shows measurements of
function generator input signal... from the first sinewave photo:
Vp-p=10.7V
Vmean=202mV
That 202mV is an offset voltage that should not exist - it may very well be the cause of the rectifier's uneven output peak amplitude. The function generator's mean voltage should be zero. Do be careful to ensure that the oscilloscope is DC-coupled rather than AC-coupled.
Your second photo of square wave from the function generator shows a similar DC
offset:
Vp-p = 9.2V
Vmean = 175mV
Your function generator should have a front-panel knob that adjusts DC offset from a negative-to-positive voltage. It should be set so that your oscilloscope shows Vmean=0V