In many ways, it is unfortunate that we can no longer discuss politics or economics in a civil way and have a full national discussion to arrive at some real, necessary national answers. It just seems that so many of us are so polarized and don't want to discuss any other alternatives. They are, sadly, topics that need careful handling.
Of course another problem would be if the new friend does not have the ability to keep the discussion within calm bounds, you have a problem on your hands.
I would never discuss someone else's weight unless they brought it up, that's for certain.
As for what someone paid for the trip, that too can cause problems only because there are so many different rates for everything. We made our reservations very early, got some early savings, and some others have come along the way. I'm not sure I even know what the current number is for our Med cruise in October. You might be very proud of yourself for having found a way to save x dollars, but the person you are bragging to about it will not love you for it. The same is obviously the case for airline fares these days. I might tell people where I found my fares, but a comparison isn't going to make one party to the conversation very happy.
The OP does state one good method -- asking the other person. If you are not telling them about yourself, but rather asking about them, you will make better friends.
Oceania REGATTA - Scenic Panama Canal - Dec. 5 - Dec. 21, 2014
Uniworld RIVER AMBASSADOR - European Splendors, June, 2013
Disney DREAM - 4-Day Bahamas - January, 2013
Oceania RIVIERA - Accent on Italy - Oct. 24 - Nov. 3, 2012
MV ECLIPSE - 7 days in the Galapagos - June, 2012
HAL STATENDAM - 7 day Glacier excursion - June, 2010
Disney WONDER - 3-night Bahamas - Dec. 27-30, 2009
Oceania REGATTA - Scandinavian Splendors - Aug 8-22, 2009
Carnival FANTASY - 3-day 1990 or '91
USS HANCOCK (CVA-19) Two WestPac cruises 1969-71 Oops those weren't pleasure cruises!