It's uncertain, after looking at closeups of these images whether or not there is really a problem.
The first thing I would test with the camera is whether "infinity" focus is indeed a pinpoint sharp focus or slightly beyond that. On the other hand you are drawing an assumed more or less circular object (star) with a square medium (pixels) so stars of different sizes in appearance will not be defined with a perfect number of pixels in terms of simulating a circular object. Any slight startrail will then result in an object which is less than perfect in the representaion you might expect.
A film photograph is made with tiny more or less circular globs of silver halide of varying size and density so there is an "averaging" effect which shows less distinct boundaries thanthe pixel. This tends to make tiny objects appear with smoother boundaries, especially as you would see on a negative when looking at great magnification. Because pixels are delineated by numbers in a matrixand thus of square or rectangular proportions they tend to "stairstep" especially when viewing at the pixel level. Once such an image is made into a print, there is a great ameliorating effect due both to the print mechanism and to the interpolation process (depending on the print engine used)....
I would first examine whether or not infinity on this particular lens (camera in this case) is truly a pinpoint focus - it often is not. Next I would try a few shots using manual focus and backing off a bit from the full infinity setting. This "may" be all you need to get the results you expect.