Simple reduce the quality setting. I just tried a test export from Lr, using my usual POTN export settings, just changing the size to 1920 on the long edge. So it was JPEG, sRGB, Quality setting 80 and I limited the EXIF to Copyright and Contact details. The file size was 802 KB. Most of my 1280px long edge images, with full EXIF are smaller than 500 KB. [IMAGE'S LINK: https://flic.kr/p/Zh5S1y]de Havilland DH 82a Tiger Moth
If you leave the quality setting at 100 then you get no compression, and comparatively huge file sizes. I have done quite a bit of testing on JPEG compression, and it is not possible to see any difference in quality between them. In my test I found that if you compare an 8 bit TIFF, Q100 and Q80 JPEGs in Ps there is a measurable difference between all three versions, the differences between the Q100 and the TIFF, and the Q80 and the TIFF are different, but they are of the same magnitude. It is just different pixels that have the slightly different values. There is actually a slightly bigger difference between the two JPEG files.
Although a maximum quality JPEG file with no compression is about the same size a an equivalent 8 bit TIFF file, and maybe even larger on occasion, you still get JPEG conversion artifacts. This is because JPEG doesn't use RGB to store the data. Instead it uses a luminance channel and then two chrominance channels. It is the colour information that is mostly compressed in JPEG, especially in the higher quality settings. Reducing the colour this way is pretty much invisible.
I found that Q80 in Lr was optimal, since quality wise you can't measure a drop from maximum quality, and it generally reduces the file size by between about 60% and 40%.
This was my test image for the 1920px export. It seemed to offer a reasonable mix of detail etc.
by Alan Evans
, on Flickr