As I said. Technology has been marching on.
But even with light-guides (to guide the light onto the photodiode), there are still limits as to much you can shrink pixels.
These are physical entities and you cannot shrink them indefinitely with a given technology.
The light guide cannot have a diameter zero, which is obvious even from the picture you posted - if your keep shrinking the pixels.
You make it sound as if smaller pixels are always better - and that's not unconditionally true.
That's the only point that I'm making.
There's a physical limit that cannot be crossed.
That's why manufacturers are using finer and finer CMOS processes (Panasonic is down to 65nm now).
And also looking for alternative solutions - like BSI, Sony's stacked technology, etc..
So, smaller pixels are generally better - but only when newer, more advanced technologies are used.
There's also the issue of the full-well capacity of a photodiode.
Smaller full-well capacity automatically lowers SNR. You should know that.
So, it's a balancing act, really, for pixel engineers.
A blanket statement like 'smaller pixels are always better' is just that - a blanket statement.
Some necessary small print needs to be added to discussion .