for a previous discussion on this topic.
This is due to newer computers using higher resolution screens, which make the user interface smaller. For older applications, such as older versions of mIRC, that are not high resolution/DPI-aware, Windows magnifies the interface, including the buttons. This makes the buttons larger but the interface can be blurry and there are many aspects of the application that will not work correctly.
This is on my to-do list. However, it will require a professional icon designer. There are around 70 icons and, the last time I looked into it, the cost was significant. As mentioned in the previous thread, all of the icons will need to be updated to include at least 16x16, 32x32, 48x48, and 256x256 sizes, and will likely require a few other intermediate sizes