The reason that the 7100es weighs substantially less than your old 777es is because it uses digital amplification as opposed to analog. It still has a massive toroidal transformer but since the digital amplification is something like 90% effecient, it doesn't produce heat like analog amplification does and as such, does not need the massive heat sinks that make up most of the weight in traditional analog receivers.
For what its worth, the 7100es is a lot more powerful than even the 5000es from a couple of years ago thanks to its massive power supply (the old 5000es used switching power supplies and had no toroidal transformer.
With all that said, the 7100es will sound completely different from your 777es as it has a smooth digital sound which is very revealing and clear. Your 777es by comparison will sound more warm in not a touch muddy by comparison. You may find that the digital sound is not for you. Take a listen and see.
For what its worth, the 7100es is a lot more powerful than even the 5000es from a couple of years ago thanks to its massive power supply (the old 5000es used switching power supplies and had no toroidal transformer.
With all that said, the 7100es will sound completely different from your 777es as it has a smooth digital sound which is very revealing and clear. Your 777es by comparison will sound more warm in not a touch muddy by comparison. You may find that the digital sound is not for you. Take a listen and see.