I had no idea about what happens before (or after) a bout. Here is a link to a YouTube page for a video (with sounds) of scenes before the action of a Grand Sumo Makunouchi division's bout; the two sumo wrestlers seem to be Kisenosato and Hakuho.
https://www.youtube.com/watch?v=CosZTixmDsYYou see
a caller call the name of each sumo wrestler (though it almost cannot be heard),
the referee introduce the bout (, turning his "gumbai" in two directions successively),
sponsor banners go around the ring (in two groups, from around 1' 45'' in the video)
and also a part of the preparatory ceremony performed by the two sumo wrestlers.
A banner is 70 centimeters in width and 120 centimeters in height.
The reward money is set for a bout only of the Makunouchi division.
The amount which a sponsor pays is now 62,000 yen (with consumption taxes included) per banner.
A winning wrestler gets 56,700 yen, the half of which goes to his stable.
The rest (= 5,300 yen) is paid as a handling fee.
one yokozuna who won, got a stack of envelopes that was a bit bigger than the stack an ozeki got for winning an earlier bout, but still that stack was a lot smaller than the one that Hakuho got when he won Is anyone able to help me with the logic behind this, please? Well, the reward money is set for a bout itself, and the winning wrestler takes the money regardless of which wrestler wins the bout.
So, when a popular yokozuna or ozeki in his good conditions happens to be defeated, there may be a case, in contrast to your examples, where a winning lower-ranked wrestler receives the money in a bunch of envelopes.
When a scheduled bout is not practically made because one wrester has withdrawn, the reward money is paid back to the sponsor.
A sponsor can have one short commercial message (which includes its name or brand) per banner printed on a sheet of the list of daily bouts (which is handed to each spectator) and announced in the stadium.
(Trivia: In NHK real-time TV and radio programs, NHK turns off microphones set to catch the sounds inside the stadium during the announcement, so that commercial messages are not heard clearly in the broadcast).
Oh, I just got reminded of a ceremony called "yumi-tori-shiki" which is performed after the last bout.
Traditionally a sumo wrestler who has finally won a competition performed this ceremony on the last day, using a bow ("yumi") given to him a prize.
Nowadays this ceremony is performed daily by a sumo wrestler who has learned the manners, instead of a wrestler who wins the last bout on the day.
https://www.youtube.com/watch?v=88FrG_PKl6g