why is my 8minute iMovie using 15g of storage and causing severe lag in the program?

Ive never used iMovie before but started playing with my photos and videos and was happy with how easily i could blend them together with music from iTunes and cute little transitions. The frustration came when I started running out of space on my macbook pro. I had around 15g of storage left and after about an 8minute video, was getting notifications that my start-up disk was almost full. WHAT?!?!?! these are all clips and vids imported from my iphone. Im not too technically savey when it comes to stuff like this but how is it that a few small photos and movie clips blended with a little bit of music is eating so much of my computer's storage? Also, the program became very laggy and started to disfigure the top status bar. All the words are messed up and jumbled. Why am I having this issue???? It’s extremely aggravating!!!!!