This problem has been stressing me out for so long. Lightroom just can't seem to export my RAW files to JPG at full size anymore. For years I would upload raw files at around 24mb and the jpg full size export would come out to a similar size. Nowadays full size jpg exports come out at around 5-10mb. I don't know what has changed but this problem occurs on my desktop classic and laptop CC versions of lightroom. I have made sure I'm not trying to export off smart previews and I have checked my export settings a million times so I have not made any export constraints in terms of file size or file dimensions.
What quality setting are you using during the exports? There can be a huge difference in output file size when comparing files exported at 100% versus something like 80%, and of course the actual image composition can also affect how efficient the compression algorithm can be. A couple of examples:
1. A 45MB file, fairly detailed (bookcase), at 100% a 33MB file was produced. At 90% it was down to 18MB, and at 80% it was down to 12MB.
2. Another 45MB image, this being the moon in a black sky (so less detail, thus more effective compression expected). At 100% the output file was 25MB, at 90% it was 16MB, and at 80% it was down to 10MB.
Generally speaking, I would never expect the output jpeg to come out at a similar size to the original, and to be honest as long as I have the export settings setup correctly, the size of the output jpeg is something I would rarely focus on. As shown, there can be huge variation (even with full resolution exports) depending on the quality setting and the image content.
Check your settings again, and provided they are correct (especially the quality slider) I really wouldn't worry about the physical size of the output file.