Cait,
First let’s find out what bit rate you encoded your sings in.
Open iTunes, select a tune and get info on it (Command-I). In the Summary screen take a look at the Kind, Size and Bit Rate info.
For most cases, MPEG Audio files are fine. I usually encode at a bit rate of 192 (matter of taste). This will result in small file sizes.
If, on the other hand, you see info like Apple Lossless audio file you can get bit rates around 862. This will result in BIG files around (and larger than) the size you describe. It sounds like you may have ripped your CDs in this format.
For future use, open up iTunes preferences and select the Advanced tab. Select Importing and under "Import Using" select MP3 Encoder from the pop-up menu. For Setting choose one of the three presets (Highest being 192) or Custom to configuring things as you like. Go ahead and click OK to exit the preferences.
Now, as far as those files you've already encoded, IF you find that you DID encode as Apple Lossless, select the files (you can select as many at once as you like) and then select Convert Selection to MP3 from the Advanced menu. This will re-encode the selected songs using the settings you just entered in the preferences. Once this is done, check your library and you’ll see new entries for each song you re-encoded. Find the Lossless encoded ones and delete them from your library and then re-sync your iPod.
This should do it, unless there's a bigger problem here.
Let us know if this helps.