Welcome to the forum.
File size is dependent on bit-rate, primarily. That bit-rate can be determined by the compression that a CODEC does, and then often adjusted within the dialog screen. The higher the bit-rate, the better the quality, and the larger the file size. Some CODEC's use different terms to set the bit-rate, from "Audiences," to "Connection," to "Quality." Others will just have a slider or input box, where the user sets the bit-rate. It differs from format to format.
Frame Size plays a role in the file size too, but the bit-rate is the major component by a great deal.
If your WMV's are smaller than an MP4, or MOV, with the H.264 CODEC, then it is very heavily compressed and that is accomplished by lowering the bit-rate. If your Export/Share files are much larger, it is because they are less heavily compressed, and the bit-rate is higher.
Getting small file sizes, yet at a higher quality, is a delicate balancing act, and usually requires testing. Hollywood plays this game with every title released. They try for the highest quality, and are also trying to fit the most material onto a DVD, or BD.To accomplish this, they use special software (in the US $ 100,000 range) and it's run by experts, who do nothing else, every work day. That is why they are able to squeeze a longer Duration film onto the same disc, that you are using, but the quality is higher. The material is virtually Transcoded, Frame by Frame, even though they are using about the same MPEG-2 CODEC, that you are. Each individual Frame is heavily analyzed, and also analyzed against the preceding Frame and the one following it. The comparison might be done 10 times (10 to 20 passes vs the 2 pass Transcoding scheme of PrE).
The H.264 CODEC is quite good, and is one of the approved CODEC's for Blu-Ray. It does high quality output, but with pretty heavy compression.
How do you need to deliver the material?
That will often be your guide, or at least provide the starting point for the Export/Share.
Message was edited by: Bill Hunt - Fixed typo
Thanks Bill- delivery is through the internet.
For wmv I'm using the HD720p30 preset (1280x720)
In quicktime the framesize is only 720x480 and I've been using the NTSC DV25 preset.
(which doesn't allow me to change any settings) I now see that in H264 I'm able to use
the quality slider (percentage setting) but I don't see any setting for the bitrate.
Which setting would be comprable in quality to the windows HD720p setting and a framesize of
1280x720 but without ending up with such a huge file? Thanks again for your help!
Some formats/CODEC's have actual bit-rate settings, but others do not. They are basically "behind the scenes" and what one sees is a Quality slider, or, like with FLV a connection speed. These do the same thing, but just call it something different.
The NTSC DV setting is a low-compression CODEC, and, as you state, does not offer many options for the attributes of the file, as they are set already.