problem with recording tv programms

i have a sony dvd recorder with hard drive. sometimes when i record several tv programs the last program is only recording 10 -15 min. ( 1.7 gb etc ) there is enough space left on the hard drive. it also happens sometimes when i start recording manually.
why is it so ? any help would be appreciated. thanks in advance.

edit:
recorded 4 tv shows over night, the first 2 are only 1.1 gb again about 15 minutes of recording when it should have been 1 hr and the next 2 are ok with 6.7 gb. why is it so? it doesn't make sense to me. any idea why this happens sometimes? any help would be appreciated very much.