The Export dialog includes re-sizing controls. It is likely that the number of pixels in your output, is being reduced there (if you leave these settings alone, they will simply persist from Export to Export).
Lightroom includes some preset choices for Export, but it is essential to review these, and alter them as needed. Then it is IMO helpful to update these presets, and/or save your own - so that you know exactly what you will get, repeatably, in the future
You can disable resizing altogether, or else you can set it to suit particular requirements. Also if you are saving JPG, you can set a JPG Quality that you are happy with. I personally would advise not using the option to resize to any particular FILE SIZE, but to concentrate more on achieving the pixel count / dimensions, the quality, the output sharpening and the colourspace that best suit your purpose - letting the file size turn out however it turns out.
Resolution (PPI) only makes any actual difference to the pixel dimensions, if you have specified your picture sizing in physical units such as inches. If you specify your picture sizing in pixel terms, PPI is largely incidental.
KB / MB give only a general indication of likely picture detail, and only when you compare like for like. Comparing e.g. the file sizes of a Raw file vs an uncompressed TIFF vs a compressed JPG, would not be comparing like for like.
Richard has you pointed in the right direction. The export dialog will reduce the number of pixels if you tell it to, resulting in much changed sizes.
The amount of JPG compression comes into play, too.
When uploading to my SmugMug account, I use no resizing and 90% quality for compression, resulting in megabytes of data.
When uploading to Facebook, I resize the images to 800 pixels on the long side and 80% quality, resulting in 200 KB files or there about.
Another thing that will reduce your image size is cropping during your editing.
As pointed out, the file size in bytes is a poor measure of the file. You need to be looking at the pixel dimensions.
I'll say that same with stronger words. If you are comparing the file size of photos before and after, you are wasting your time. There is nothing unusual about a JPG being compressed, because JPG is a compression technology. So 5 MB to 800KB is not unusual and is not a sign of poor quality, so stop comparing file size, it will mislead you and cause you to be at wits end for not valid reason.
The thing to compare is the number of pixels in the file. If your original file has (for example) 8 megapixels, and your export has 8 megapixels, then you are fine, regardless of the file size. Stop looking at the file size.
This makes sense. i just looked at Lightroom. Where do i change settings for import/ export resizing? when i click on import, it says no presets are applied. Maybe export? Also, where do see how many pixels are in a picture (dimensions?)
When you click on Export, there are several options for resizing:
Adjust the quality slider
Limit File Size to a set value
Resize to fit in pixels, inches, mm.
If you want to see the image size, it is show in the Metadata panel on the right of the screen in Library module. It shows both original dimensions and cropped:
In Loupe view, under View > View Options you can select data to view including file size in Pixels. The data then shows in the upper left corner on the image: