What impact does winding tightly versus loosely have on transformer performance?
See my answer here if you want the theory behind the words below: -

Normally, the flux will tend to want to concentrate in the ferrite core because that will have, by far, the lower magnetic reluctance path compared to the air between the coils and the core. The reluctance of the air is in parallel with the reluctance of the core and just like an electrical circuit comprising parallel resistors of vastly unequal values, the current flows in mainly the lower value resistor. That lower value resistor is akin to the core’s reluctance.

So, the flux congregates in the lower reluctance path of the ferrite core.

Now, if we test this out by expanding the radius of the coils, we see that the reluctance of the core remains the same but the reluctance of the now "fatter" air (a parallel component) gets smaller. It gets smaller because there is more air "area" and, of course, reluctance reduces with area.

If you take it to greater extremes, you’ll see a reduction of the percentage of flux congregating in the core and you'll get leakage inductance. This is because the air path is only really useful for generating local lines of flux coupling only a few turns of the primary winding AND importantly, those local lines do not couple to the secondary. Flux not coupled means leakage inductance.

At this point, the unloaded secondary voltage will be noticeably lower due to the overall rate of change of flux being lower. If the secondary were now to be loaded, the situation would become worse because the primary referred load current (due to the secondary load current) is passing through the primary leakage inductance and, effectively lowering the voltage seen on those primary turns that can be said to be 100% coupling the secondary.



The above is the equivalent circuit of a transformer showing how leakage inductances lower the voltage seen at the heart of the circuit (the ideal transformer).