My video cam jvc mg 630 is taking too much mega bytes. is there a way to adjust it to less bytes?

i have read the manual to adjust it but i did not see how to adjust it so that it will take only (for example) 10 mb for 10 minutes video, instead of 20 mb... what happens is that i cannot save my videos because it takes up much of my hard disks. i dont want to store big bytes in my hard disks. Is there a way for video cam to be adjusted? i am so beginner i dont understand much technicalities. thanks.