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A file is more than just its content. There is the compression system (codec), quality level, bit rate, etc. Even the brand of codec can make a difference.
It's pretty much impossible to get the same video size out as you put in. (Well, unless you're working with DV-AVIs, which are compressed at a fixed rate.)
As Steve points out, there are two parameters, that affect File Size:
With a fixed Duration, the only way to adjust File Size is lower the Bit-Rate (lower Bit-Rate = smaller File Size, but at the expense of quality.
Now, some CODEC's will allow one to use a lower Bit-Rate, but still retain perceived quality, better than some others, but it is still the Bit-Rate, that affects the File Size.
Some Export/CODEC options will list the Bit-Rate directly, and MB/sec., where others will instead have a "quality" setting, and the Bit-Rate's MB/sec. will not be directly shown, but will relate to some quality setting.
One major consideration is what is to be done with the resulting output file. If it is to be used for additional editing, then I feel that File Size is not a big issue, as I want the ultimate quality. If one is delivering the output file, then there are many considerations, from the platform of the recipient's computer, to the player being used, to the necessary quality of the file.