the megabytes of an exported JPG are meaningless. Do not use the megabtyes to determine there is something wrong. You have selected the proper settings in Lightroom, and as far as I know, Lightroom gives you what you asked for and so the problem (if there is one) cannot be identified from this information.
You'd have to show us an example of zooming that produces poor image quality. What software are you using to do the zooming? What magnification are you zooming to?
Was the original cropped? If so, what is the size in pixels (width and height) of the cropped original before export?
The file size in megabytes is not a good measure of image size.
For compression, 80 is a good number; I tend to use 90; 100 is wasteful.
Image size is all about the number of pixels, length x width. Since you do not have "resize" turned on, things should be good. How much cropping are you doing?
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Setting the Quality = 100 is useful if you’re testing to see what might be wrong with an export to eliminate the Quality as the problem, but routinely using Q=100 is wasteful, yes.