Howdy,
it´s really more massive and more than pictures may tell; as usual with rectangular watches, the diameter (and substance) does grow exponentially. So it´s not just two mm more, but really a lot "bigger".
Well, i really love the old 25990 and one of these is my preferred daily wearer.
I find them to be a perfect fit on the wrist, but the black dial with bright polished hands at times can be difficult to read.
The new 25820 does a lot better in this regard.
On the wrist, the new square shape results in a different placement on the wrist.
Due to the wider case, it doesn´t "move" on the wrist as easily, which is supported by the wider strap at the same time.
It´s very comfortable and not too heavy, but a nicely balanced feeling.
What i don´t love that much is the subdials to be a little more dominant due to the wider case.
With the 25990, the subdials are located more towards the outer edge of the dial, which naturally isn´t possible with a wider dial.
But that´s a subjective matter of preferences only.
So all in all, if you´re fine with the 25990 i´d easily expect you will find the new 25820 very comfortable, even if it´s a little more massive and larger!
Cheers,
Peter