Thank you Terence for replying.
First of all, I imported my Canon pixs using a card reader to my MacBook.
Then later... (I don't quite remember exactly what I did but here are two possibilities)
... I most likely exported photos and videos from iPhoto on my MacBook to a USB memory stick, then importing the data to iPhoto on my iMac.
... The other possibility is that I selected the photos and videos in my MacBook's iPhoto, selected View in Finder, moved or copied them to a USB memory stick then imported them to my iMac's iPhoto.
The result was that...
I had obtained pictures of each video but no movies attached.
Does this make sense?
I still have all my videos and photos on my MacBook, luckily, so I can try the proper way of transferring from one Mac to another without losing the videos.