As Aaron explained, it is a simple matter of math. That is, while doubling, tripling, etc. results in each pixel simply being enlarged (each pixel becomes a block of four, nine, etc. pixels), scaling by 1.25x, etc. gives fractional results.
For example, if you want to magnify an area of the screen that is 10 pixels wide, doubling it is simply a matter of making a 20-pixel—wide area and copying each source pixel twice. But what happens if you want to make it 1.5x? You have to make an image that is 15 pixels wide, but since there are no half-pixels (sub-pixel rendering notwithstanding), you have to double every other pixel.
Now imagine trying to zoom to 175%. You would have to make an image that is 17.5 pixels wide. Wait, what‽ How do you make a 17.5-pixel—wide image? You can’t! You have to round it to either 17 or 18 pixels wide, then double only some of the pixels using some even more messy formula. This makes the result even more unpleasant to look at and difficult to read.
Your best bet is to use a different program which uses advanced math to scale it nicely, similar to how emulators often employ different filters to scale low-resolution video-game consoles to high-resolution monitors.
Sadly, there don’t currently seem to be any Windows zoom/magnification tools that support both fractional zooming and advanced filtering (my go-to tool, Xoomer turns out to not support either). I do know of a couple of a few programs that have potential:
- Mooz has a nice, minimal interface with good results, but zooming doesn’t seem to work, so I cannot tell if it can do fractional zooming.
- On Screen Magnifier has lots of fancy features including numerous filters that can make the results look smooth and great, and it does have fractional zooming, but only for down-scaling! That is, shrinking to less than 1x (which no other magnification tool I can find does).
- Virtual Magnifying Glass has a lot of features as well. The only fractional zoom level it supports is 1.5x (which is still more than other tools), and while it looks much better than the Windows Magnifier at 1.5x, it could be better with some filtering).
While none of these quite fit the bill, the bright side is that the above tools are all open-source and available on SourceForge, so you can easily put in a feature request to have them add the necessary features.