If you don't need line outputs, mic input, or style creating/importing, the GoKeys3 is even more budget friendly; but keep in mind both the Gokeys 3/5 are very, very modern and limted when it comes to styles. But they do sound really good.
If you don't need arrangers at all Casio actually makes an 88 key stage piano (PXS1100) that's only 23 lbs (but that's only 2 lbs less than your s770). They also make arranger 88keys that are the same weight, PXS3100 and CDPS360.
But if you just want piano, nothing else? Without line outputs, the CTS1k (9.8 lbs) has 40 'bread and butter' sounds. no arranger. But for $30cdn more (about $20usd more) you can get the same thing with an arranger, the CTS400. The CTS500 above adds line outs and filter knobs.
The PSRsx600 mentioned above is only 18 lbs, and covers a *lot* of what the s775 can do.
Finally, Yamaha themselves has lighter NP series, NP15 and 35, that are 61 key and 76 key boards with only 10 sounds. However, the choice to put the speakers on the sides instead of above or even underneath, makes them both much wider than you'd expect (their 76key is about the same size as the Casio 88, and the 61key Np15 is roughly the same width as a PSRE400/410/425
Mark
www.youtube.com/MarkWilburnTLM/Videos
DX7, CLP300, PSR60, Roland E20, CVP309, PSRS970, Fender Strat, Ibanez SA262, Yamaha CG131, Fujiyama FC390, TUC-Kitty