Importing iPhone movies increases file size 3-4x

Question

I imported clips from various sources into iMovie11, then I compared the files in the iMovie Events folder to the original files.
1. AVI movies are identical to the original files (file size and picture size - 640x480), regardless if I select "large 960x540" or "full size" when importing. So far so good.
2. However, Movies that were taken by iPhone 4 (MOV, original picture size 1280x720) end up 3-6x larger.
    2.1. If I select "full size" when importing, then the new file size is 6x larger than the original. (Picture size same as original, as expected.)
    2.2. If I select "large 960x540" then the new file size is still 3x larger than original.  (Picture size shrinks to 960x540, as expected).
3. Same experiment with iPhone3 movies (MOV, original picture size 640x480)
    3.1 If I select "full size" then the file size is 3.5x larger than original.
    3.2 If I select "large 960x540" then the file size stays the same.
   (Picture size stays the same, 640x480 as expected in both cases.)
Now this is somehow consistent with the changes in movie format and data rate:
- In the cases 1 and 3.2 above, nothing changes from original to new (AVI format "motion jpeg open dml", MOV format "h.264").
- In the case 2 and 3.1, format changes from "h.264" to "Apple intermediary codec" and data rate goes up by a factor of 6 and 4, respectively.
FPS stays the same in all cases.
Can someone explain why it iMovie does this.
No segments of the new clips are duplicated, and they are not padded with gray or garbage like I've seen in other posts. 
Thanks in advance

Answer 1

Motion JPEG is already an i-frame codec like DV or AIC so there is no advantage to converting to AIC. Each frame is a compressed JPEG.
I have some Motion JPEG that I received from a service that digitized some old 8MM film for me in 1080P. The Motion JPEG worked fine in iMovie.
The iPhone 3 created h.264 in smaller dimensions, like 640x480 (although I can't remember for sure). With smaller h.264 files, your Mac may have the processor power and memory to convert the h.264 on the fly as you render. For larger h.264 files like 720P and 1080P, most Macs do not have enough power, so you are better off converting to AIC.
So with the iPhone 3, you can make the choice yourself. You can convert them if you see a performance issue. Otherwise, edit them as is. To convert them, right-click on the Event, and choose "Optimize Media".