This is generally expected and consistent with similar systems, such as smilies.
In fact, I believe at some point they were constrained to specific dimensions, but this wasn't desirable in cases where you might want smaller/larger reaction images, or reactions with different aspect ratios etc.
The way to control the width and the height of the image is to upload an image of the correct size, which is the same as smilies. We're not planning on writing out custom CSS for image reactions (or smilies).