- Which version of LR are you using?
- What are your export settings, post it here along with the screen shot.
- In which format are you exporting the images?
- Do you apply any preset on exporting images from LR?
If you want to export the images full size, uncheck Resize to fit under Image sizing in the Export dialog.
I shoot a wedding imported with Lightroom General Presets on Auto - Tone and was very pleased with the results until I exported over 400 images to find out from my lab that the size was too small.
Can you be specific about the exact "too small" size? Are you looking at the megabytes of the export? If so, this is a meaningless number and your exports are fine, especially if you (as stated above) uncheck Resize to Fit. The megabytes will be small because JPG uses compression, its perfectly normal for JPG exports to be a lot smaller than the original.