In today’s modern military, massive things are generally frowned upon. They’re simply too big of a target.
The point of all this is, big targets are bad no matter how good at its job it is. Better to have many targets that you could throw at the enemy over and over.
Here’s the thing, I have humans but they’re incredibly advanced. Sufficiently so, that they’ve survived the end of the universe and moved on to another one. They came upon a planet where they’ve discovered magic exists. Not being sure on the capabilities of this magic; they just start advancing technology like crazy.
Starting from the medieval era, they’ve been developing technology both military and civilian. For now, they seem to be at the late WW2 to the early Cold War era. They have total air superiority and even control the major bodies of water.
Now, since everyone is so far behind on technology would it a waste for them to build the super weapons? Things such as the P1000 Ratte, the H45 Battleship and the unnamed 500-ton Japanese Flying Boat.
Reasons to build such things in a world where even a small frigate can control a large amount of coastline? A world with magic may have hidden powers that are overwhelmingly strong, better safe than sorry I guess.