Are you talking about the fact that on an LTO4 tape BE only shows 800-odd GB? That's the native capacity...not the compressed capacity at 2:1.
Otherwise, if you're only backing up to the native capacity of the tape, check below:
1. Make sure you're using the Symantec drivers for the tape drive.
2. Upgrade to BE 2010 R3 if you can. It fixed a number of issues with the previous 2 versions.
3. Check the TN below:
4. Some data doesn't compress once compressed...check the compression settings you're getting on your tapes, and make sure that compression (Hardware, otherwise Software <--that is the setting!) is turned on in your jobs.
Here are the native and compressed capacities for LTO3, LTO4, and LTO5 tape media
Backup Exec reports the native tape capacity (value on the left) of the media and not the compressed capacity (value on the right). Based on the capacities you've listed I suspect you have LTO3, LTO4, & LTO5 media in your tape library. Therefore, BE will report native capacities of 400 GB, 800 GB, 1500 GB, respectively.
The compressed capacities values are assumed @ 2:1 ratio and all tape drive/media manufactures will back that claim. The type of data you are backing up will dictate the hardware compression ratio you'll receive. Only time will tell what compression ratio this will average out to.
You should take a look at my article below to see how to interpret your tape statistics
Normally, LTO tapes are marked with their native capacity and a compressed capacity assuming a 2:1 compression ratio. A 2:1 compression ratio is hardly achieved in real life. Normally, you would get a compression ratio of 1.2 - 1.4. See my article below on the subject of compression