Why the overlay dimensions not fixed always in FFMPEG

i am trying to overlay image above another

backgroundImage 2448*3264

-i $backgroundImage.jpg  -i $overlayImage.jpg -filter_complex "[1:v]scale=500:500[ovrl];[0:v][ovrl]overlay=0:0" $outPutPath.jpg

outPut

enter image description here

the overlay dimensions look smaller than 500/500 !! besides when i use the same previous command but with difference backgroundImage dimensions which is 736*272 the outPut look like following

enter image description here

the overlay became huge and not the same pixel in first output ?

if i am not wrong that’s because the difference between backgroundImage dimensions

How can i prevent this act? i want like to use fixed result for example overlay value lets say 500*500 and i want the overlay look always the same whatever the backgroundImage dimensions is

so According to my logical desire, i want something like following if the scale was 300*300

enter image description here

enter image description here

the overlay always the same in dimensions .. I apologize for the inaccuracy of the drawing

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