Step up-down transformer tripping circuit breaker
All transformers are essentially a "short circuit" for the brief instant that they are energized, because it is the interaction of the magnetic fields that impedes (resists) the flow of current, and for a brief instant, there is no magnetic field yet. This is called "inrush current" and can exceed 10x the rating of the circuit breaker. Often that inrush is of such short duration that it is too fast for the breaker to react to, but if it is TOO high, then it can trip the breaker. Because of this, MCBs, like the ones you show, can be purchased with different "trip curves". The ones in your photo are "C" curve, which are for general purpose use. For feeding a transformer, you want a "D" curve (so it would say D10 instead of C10). A D curve breaker will allow up to 14x the rating for the instantaneous trip function, which is designated for handling transformer inrush.

When the smaller transformer worked, it was because >10x the transformer inrush current is still  less than the breaker instantaneous tripping point. You could get a similar effect by using a larger breaker, but then you would have to change the wire.