Make sure you didn't download the High Definition (HD) versions of the series. You can check by right->clicking on each of the end, choosing Get Info, and looking at the Video Quality field under the Summary tab. It should say whether it is in Standard Definition or HD.
To prevent HD versions from being downloaded in the future head to the iTunes Store, click the Purchased link, and then select TV Shows and remove the tick mark next to Download HD when available. You should be able to download the SD version now from under this same page.