it's because there is more wastage on a smaller panel and it fundamentally costs the same as a larger panel to make
65inch is the optimal size for Their process, which is why it's priced so competitively Vs the 55inch and sees larger discounts.
It's also why the bigger panels are crazy expensive.
an example image of how the manufacturing has changed over time
This will continue and no doubt it is why they have chosen to not release a budget BX version of the 48inch model
I wonder if it might have a knock on effect of making the range ever so slightly more expensive too as they slot it in