I am trying to understand what kind of video files Shutterstock actually delivers to a buyer. I realize you just upload your high-res files for video and they do the rest, but I am looking at the buyer video download options on a video on the site and it says the following:

1 - On the 1080 HD size video it says it is an MJPEG file, but if I am not mistaken that is just the Codec being used and is not saying anything about the container or file format. Would that be fair to assume it is a MOV file that was created with the MJPEG codec?

2 - On both the Web and Low Res file size options it says that the files are MOV, but I am looking at one video as an example which is 30 seconds long and for the 852 x 480 video size it says the file size is only 24.8MB. That seems very small for a MOV file to me of a video of that length and resolution. Does anyone happen to know which codec they are using to create these MOV files that brings the MOV file size down so low?

If perhaps Shutterstock has a link somewhere that explains all of this that would be great to see too. But I did some Googling and didn't find anything I am afraid.

The container is mov.A codec is something else.
Everything ss downconverts is a flavour of h264 thats why you see such a small file size.
My opinion is forget about their downconversions they will be irrelevant in the future (seriously h264 as a professional format?) and
upload your work as prores or as mjpeg in a mov container at 90% quality  at the very least,unless you have serious bandwidth restrictions.

I think they accept avi and wmv containers too but mov is more universal as a container,so stick with that.

Maybe this helps.http://submit.shutterstock.com/tutorial/
Also they have a sticky post on their forums (i think)

Agree.

Why mjpeg and not ajpeg? I always though mjpeg is best for interlaced video and ajpeg for progressive video. Most videos these days are progressive, certainly those from a DSLR are progressive.

No idea what ajpeg is.Mjpeg can be interlaced as well as progressive.
I dont do mjpeg except for a few times to upload to cc,so the only flavours of it available on my encoders are mjpeg1 and mjpeg2.To be honest i consider this a crappy format as well,but it is compatible with all nle's and offers a good quality/size ration,and agencies prefer that.
But my advise is go with proresHQ or plain prores even if you shoot/finish at 8bit.Everyone can export prores at this point even for free.

My appologies. It should be Photo Jpeg vs Motion Jpeg. 

From Shutterstock:
PhotoJPEG - Non-DV progressive scanned video. Use the 75% / high quality level.
MotionJPEG - Non-DV interlaced video. Use the 75% / high quality level.

Oh i see,but nonetheless disregard these guidelines theyre almost archaic.
But if you want to use these use mjpeg as it works for both video sources but not at 75%.Seriously this is not 2005.

I was curious and looked at the guidelines of Pond5. This is what they say:

Exporting to .MOV

We prefer HD footage to be saved ProRes 422 or in the photo-jpeg format, which is a low-impact, high-quality quicktime container and can be exported from most editing and conversion programs. If you're not sure how to export photo-jpeg files from your editing programs, just export a full-quality uncompressed .mov, then run it through the Mpeg Streamclip workflow to make a file for our collection.

No mention of Motion Jpeg. It is not even listed in their list of accepted codecs.

Then do a pjpeg and an mjpeg export, compare size and quality, and go with which suits you best.