I have hundreds of JPEG photographs which were scanned about 5 years ago from negative using a Konica Minolta DiMAGE Scan Dual IV. The dimensions are ~4500x3000, and the filesize is around 12Mb, compared to shots from a DSLR with dimensions of 3000x2300 and filesize of 2-4Mb (actually, these are the output from a RAW convertor). The filesize is obviously quite a big difference, but the issue that's bothering me is that the (perceived) loading time is at least 10 times slower.

Is this size/speed discrepancy likely to be because the scanner software saved the JPEGs inefficiently / using an old compression format, or is it simply that the scanned negatives contain much more "detail" (in the form of grain/noise) than the digital images? If the former, is there a way to losslessly optimize them? I've tried re-exporting the scanned files to full size JPEG from my RAW software but the filesize is pretty much the same.

Both files will have been saved at 100 quality.