Phase problems are not due to the size of the piano but to the different distances of the microphones (small capsule pair and large capsule) from the sound source.
The large capsule captures low frequencies better and is placed at the tail specifically for this purpose. At the tail, high frequencies, which are very directional, are less noticeable, and a mostly dark spectrum is captured (which is what the small capsule lacks).
The h4n is a toy microphone by design; it is not made for studio recordings but to capture decent audio "on the fly," however, you conveyed the idea well. Obviously, if you move the microphones closer to the soundboard, you will hear more of the effect of the hammer hitting the string. These are things that can be partially corrected using a compressor with a very fast attack, but if you have the chance to move the microphones further away, even better. The gold standard in recording is to try to manipulate as little as possible in post-production. The less you manipulate, the better the capture.
When the microphones are moved closer, the AB technique no longer makes much sense because they no longer create spatiality if they are in the immediate vicinity of the strings. In that case, an XY technique with reverse panning is better. Capsules between 90 and 135 degrees.
However, for classical music, you need to stay quite far away. Close miking and XY are more suitable for Jazz piano.
Regarding the large capsule at the tail, evaluate whether to place it just above the soundboard line or right under the instrument, aiming at the soundboard from underneath. It depends on the microphone you use and its frequency response. With the Neumann U87, if I placed it underneath, the bass came out too muddy; conversely, with a tube CAD 8000, I had to keep it underneath because, given the bass strings, it would blow out too much. You must experiment and, above all, evaluate distances. With my microphones, I now know how to behave because I know them; with other microphones, the distances and positions are all to be experimented with, while still remaining in AB for the small capsule and using a microphone at the tail for the large capsule.
Once you know the basic positions, experimenting with distances requires time and trial after trial, but in the end, you will see that you find the right positioning.
Consider that much of the work is done by the room, especially in classical music. In the great recordings you hear, there is a lot of room; therefore, if you record in a small environment, you cannot expect results like those on a record, although you can tweak it in post-production. To make records, you need microphones costing 2,000-4,000 euros each, concert halls, and an entire professional audio chain that costs thousands of euros, but this does not mean you cannot achieve more than acceptable results even with more modest equipment.
