Photo dimensions and quality seem random after export to JPG from Lightroom

I'm fairly new to Lightroom (I have the latest standalone version, not CC) and have been shooting with a Sony a6000 in raw format. I import my photos without copying to DNG since LR accepts Sony's ARW raw photo format. I then make my edits and export the photos in JPG format. Looking at each photo's properties, I see some photos retain their original 4000x6000 dimension and others (seemingly randomly) get reduced to a dimension of 1080x1616 and the resolution and quality of the photos suffer. I want to provide photos to clients in such a quality that, if they wanted to, they could make a large canvas print out of any given photo. I like to check the photos using an online service that ends up giving an error when picture resolution is too small, and the photos that get downsized to the 1080x1616 are not a high enough resolution to be blown up.

Does anyone know what might be causing the dimension and resolution to be reduced? Is it a type of edit I'm doing to the photos? My file settings are set to JPEG at 70 quality and is not limited to a file size. My image sizing settings have resize to fit unchecked and 240 resolution. Also, I'm not resizing/cropping the photos during editing. I can't figure out why some photos seem to be downsized while others don't even with these blanket, static settings.

Any help would be so appreciated! Thank you.