When it comes to upscaling an image (making it bigger), your image editing software will try to calculate the color of each new pixel that gets added to the resulting image to make it bigger. These new pixels are calculated using one of many techniques, most of which add new colors or blurry pixels close to the edges of your sprites. This is normally not the desired outcome when it comes to pixel art. With pixel art, you usually want clean edges and to keep consistency with your color palette.
"Nearest neighbor" is the name of one technique that only copies the color of the closest pixel to create a new pixel when upscaling. This results in the desired effect of clean edges (not blurry) and an "unbroken" color palette.
You can read more about scaling algorithms here.
Now, I believe that for your game you may have used the scale property of your enemy objects to make them seem bigger. If this is true, this means that your game engine is scaling your sprites with one of the other techniques. To fix this, I believe GameMaker Studio 1.4 used to have a global setting to turn texture interpolation off, so there should be something similar for GameMaker Studio 2 if that's what you are using. On the other hand, if you are scaling your sprites before importing them to GameMaker, your image editing software should have a dropdown menu when using the scaling tool that lets you select what kind of interpolation to use. Simply select "nearest neighbor" when scaling pixel art.
Now, here is the thing, to keep consistency with pixel art you want to have the same scale on all sprites/images regardless of interpolation algorithm used. So, either scale everything the same amount, or keep the scale at 1.0 on every single sprite.
For example, for my game, I chose a game resolution of 480x270 and I drew all sprites according to that size; the cow sprite is 36x24 for example. All sprites are 36x24 but the entire game viewport is then scaled to the right window size keeping the aspect ratio during gameplay.
I recommend you watch this video about Inconsistencies with game art.
Also, there is this series of Pixel art classes by AdamCYounis that you can watch too.
I recommend both of those channels btw.
Hope this helped a little.