Make sure soft shadows are not enabled in the sky lab. If you want soft shadows, change render settings to premium and rpp to 32, and re-enable soft shadows in the skylab and or on any lights you may be using. The filesize is correct. Is 3 MB really that large? The photos I usually work on for school range from 130-300MB each. When you are done you can save a version as a jpeg that will be about 120K, so don't fret if you're worried about uploading to the web.