Hello. My name is***** can help you with your question.
This could be happening for a couple of reasons. If you are recording to certain manufacturers DVDs, they may have a lower capacity than that which you are used to. But more likely, you have the record mode set to a shorter but higher quality record mode.
You can adjust these recording modes and links of recordings in the settings menu. The option is called recording mode and it has 2 in 3 letter abbreviations to describe the recording mode and each one has a certain length of time that will record for example Excel or LP. You can see these modes in page 34 of the user manual which I am giving you a link to here.
Please let me know if you still have questions.