Let me spam you another answer: Svg file size more than a png image
Here is a graph where the x-axis is the pixel size and the y-axis is the file size.
A vector file is mostly independent of the dimensions and a raster image is proportional to it.
But that is not the full story.
Now let us think in an uncompressed raster image vs a vector file ...
An .abr file is not an image file.
It's a brush definition file for Photoshop.
This article explains how to use Photoshop to export thumbnail images of the brush definitions. But without Photoshop, there's not much you can do with the file.
If you are seeking merely to preview the brush tips in the .abr file.. there are many small freeware or shareware ...