There are a lot of issues you get from scaling an engine up, one of the major issues being combustion instability. The larger the "blob" of fuel and oxidizer mixed together that you're trying to burn, the more you will get things like eddy currents and shockwaves flowing through the material. These disrupt the combustion and make it uneven, possibly leading to feedback cycles where these elements self-reinforce eventually causing vibration or otherwise damage to the engine. The smaller the physical size of your combustion chamber the easier these things are. The largest single engine ever built, the F1 engine that pushed the Saturn V, had massive issues with this that were further made worse because the engines were largely designed without the aid of computers.
