The entire file will be downloaded, or loaded by whatever application you open it with.

For example, I appended ~45K of 'AAA' to the end of the first image below using

copy 4344.jpg/b+con junk.jpg and just holding down the A key for a few seconds.

2005-09-19  09:47            15,996 4344.jpg
2007-07-25  16:25            60,532 junk.jpg
[download]

When I load the first one into ifranview it tells me

Disk size: 15.62 KB (15,996 Bytes) Memory size: 239.09 KB (244,828 Bytes) Loaded in: 15 milliseconds

When I view the second it says:

Disk size: 59.11 KB (60,532 Bytes) Memory size: 239.09 KB (244,828 Bytes) Loaded in: 16 milliseconds

Also, I took the largest jpg I have (19MB) and copied it to another file twice. When I load these two I get

18.92 MB (19,838,175 Bytes) 667.42 MB (699,840,040 Bytes) 8891 milliseconds

37.84 MB (39,676,350 Bytes) 667.42 MB (699,840,040 Bytes) 9734 milliseconds

So the on-disk size is just one (small) part of 'performance'.

If this is critical, then you should be more concerned with matching the compression level, whether they are interlaced, the numbers of color table entries, and the final expansion size (essentially X x Y x depth) because these will all have a much greater impact on display time peformance, and memory usage, and the time taken to allocate the memory, than the time taken to load off disk will.