The most obvious similarity is ofc their names, mouse and big mouse.
Big mouse and mouse are both involved with crimes.
Although the characters are different they both deal with 'criminals'.
Mouse has a lot going on in each of their episodes and that makes you question everything, also there are a lot of plot twists that make it really interesting.
Since I watched mouse (in may 2021) it has been my all time favourite kdrama, but now that I watched big mouse it almost reached mouse (while I watched a lot of kdramas in between).
But still mouse is better than big mouse in my opinion.
If you loved big mouse and are a thriller fan, I really recommend mouse.