What ever Udvar-Hazy has in his mind. An A380 shrink it is not.
GOOD LORD! 19 frames!
Please take a deep breath and relax.
Does anyone happen to know the MTOW for the -900 variant?
No, because it hasn't been developed, and doesn't look like it will be any time soon. There may be some internal numbers floating around airbus, but even then those would be design studies rather than firm configurations.
Will it actually be physically larger than Antonov's 225? Right now I think it's smaller but heavier, isn't it? Which I don't understand since A380 is made of plastic and An-225 is a solid cast-iron molding. How is that possible? Or is it the other way round, A380 is bigger but lighter? That'd make more sense.
The An-225 has a length of 84.0m, a span of 88.4m, empty weight 285,000kg and MTOW 640,000kg (raised from an earlier 600,000kg). It is very definitely not made of cast iron (goodness knows where that idea came from?) but rather aluminium.
The A380 has a length of 72.7m, a span of 79.8m, empty weight 277,000kg and MTOW 575,000kg. It is very definitely not made of plastic (again goodness knows where that idea came from?) but rather primarily aluminium, with a reasonable number of composite components.
I would warmly encourage you to do a little research on these sorts of things before posting a bunch of guesses.