Although you haven't said so, you are apparently exporting jpgs. All jpgs are compressed, even when the quality is set to 100. Moreover, the amount of compression applied varies according to the amount of detail vs. solid colored areas in the image, so a highly detailed, high ISO (ISO noise is a kind of detail) image will receive slight compression and a low ISO image with (for instance) large areas of sky will compress down much more.
As elie says, exporting to jpg will lead to a smaller file and the degree of smallness is linked to the detail (and/or noise) in the raw file.
18meg to 3meg is a pretty steep reduction though with quality set at at 100 unless these are photos with very low noise and a lot of sky (or any other massive expanse of solid colour).....if the exported photos are still full size and look good I'd be really happy at such compression!
Looking at the megabytes of the export is simply the wrong thing to do, and will lead you down paths that are meaningless. I have never seen a case where a Lightroom export has the wrong megabytes, based on the export settings.
You want to concern yourself with making sure you have the proper settings in the export dialog box, and the export from Lightroom will be the correct image.
If you want to look at numbers, you want to look at megapixels of the export, not megabytes.