I've experience this, too, and it can be surprising - you're playing on a big piano, the sucker should SOUND big!
There are lots of reasons why it won't sound as big as it seems, though. For starters, the lid was probably up, and there was any kind of back board the sound would all be going out into the hall which probably sucked it up quite nicely. No two halls are alike - accoustics is a weird science. They've even taken the designs from an incredible sounding hall and built the same plans again, but the second hall didn't sound good - go figure.
Once again I will probably raise some ire, but I find Steinways to be incredibly overrated where sound is concerned. We have a couple of D's at the PSU concert hall and both are such fun to play! They FEEL great - my trills have never been better! But they sound DEAD, DULL. I really think that's just the way they are, and the US audiences and p[ianists have just gotten used to that sound.
Not to worry, though, I am sure enough of your sound made it out into the audience!