What can justify the extra size?
Also: unfortunately, you have to convert files in handbrake clip by clip.
Does anyone know of a free converter that would convert entire folders with mts-files into .mp4?
Also, for clarification: Is .mov an uncompressed file type? Why not just use .mts directly? I don't get this
In consumer video, you are never dealing with the equivalent of Camera RAW. Everything is compressed.
For uncompressed video, think of a live sporting event where there are 5 refrigerated trucks in the parking lot with the equipment needed to process the uncompressed video in real time. For Apple Intermediate Codec, you might see a ten times files size increase over a highly compressed format like AVCHD or h,264. However, with uncompressed video, you would see another 10 to 20x file size increase over the Apple Intermediate Codec file. So in your case, it would be about 100x larger than your AVCHD file. Your camera will not shoot uncompressed video, so this is all academic, but obviously, you would need a very powerful computer, you would need a large and fast array of disk drives arranged in a RAID, and you would need professional editing software like FInal Cut Pro. In practice, no uses uncompressed video except live TV, because in that format, they do not have the luxury of time for the compression. They compress at the end when it goes over the air as MPEG2.
When you are talking about converting a group of pictures compression to an I-frame codec, think of it this way.
Imagine you have a video of a duck flying across the sky. A lot of the motion is predictable. You are panning across the horizon at a pretty constant rate. The duck is pretty much the same, except its wings are beating up and down. So you could take a picture of the duck in flight, and then code some information that says, OK take the previous frame and move the background to the right, and make the wings go up a little bit. That takes up a lot less space than storing that information as a full frame. That is how group of pictures works.
When you convert the GOP codec to an I-frame codec like Apple Intermediate Codec, you are resolving those instructions so that each frame is a full frame. Apple INtermediate Codec and DV are two examples of codecs where every frame is compressed, but each frame is independent of all the frames around it.
To further muddy the water, your instinct that the camera copy is somehow better than something that you convert it to is a good instinct. This is because the first generation (what the camera sees) is about as good as it gets. Subsequent generations always have the risk of some loss of information. So I would recommend that you always keep the first generation copy. iMovie makes it easy to do this from the import screen by creating a camera archive.
One example of this is that when you convert your AVCHD to Apple Intermediate Codec, the AIC uses a 4:2:0 colorspace. Lets say that later on you wanted to edit in Final Cut Pro, where you have access to the ProRes 422 codec, which uses a 4:2:2 colorspace. I will not go into the technical differences between these, except to see that having more bits to contain color information is a good thing. And by having the first generation copy, you can convert to ProRes 422 directly, which is incrememtally better than converting from AVCHD to AIC and then converting to ProRes (which leaves some bits on the table).
To answer your second question, your workflow really depends on how you will use the footage.
My workflow is:
1) On the iMovie import screen, I create a Camera Archive. This makes an exact duplicate of the camera information.
2) I then use the Camera Archive copy to import the clips into an iMovie Event. When all is well, I will reformat the SDHC card on the camera so I can have a fresh card for the next shoot.
3) I edit a movie and export as h.264 through the Share menu.
4) I have the iMovie Projects automatically backed up through time machine and offsite (through the internet).
5) I back up the camera archive files through time machine and offsite through the internet.
6) I do not backup the Event files, because they are large and I can recreate them from the camera archive files.
7) I usually keep my event files, mainly for convenience if I want to do additional edits on a project, or if I want to use the clips in a new project. And yes, you need a lot of hard drives for this. However, if you rarely want to reedit or reuse clips once you have finshed a project, then you can delete your events. You can always create a new project by importing your camera archives again.
8) Of course I always keep the finished movies as well.