Many websites hosting these files operate on a "CPA" (Cost Per Action) model. They will promise the download link, but force you to fill out a survey, enter your email, or provide a credit card number to "verify you are human."
You download a 100mb file. You run it. It unpacks a tiny application that says "Downloading additional files..." It then tries to download the actual 15GB of data from a slow, unreliable server. This isn't compression; it's a torrent disguised as a direct download. Usually, the server link is dead, or the download errors out at 99%. Sims 3 Highly Compressed 100mb Download