During the Edo Period (1603-1868), sushi was between two to four times larger than it is today.
Screenshot: ルトロン - おでかけ動画マガジン
But why? One of the biggest reasons is that Edo (present-day Tokyo), where sushi was invented, was a metropolis from which large numbers of people were passing in and out. They needed food they could take with them, and sushi was the era's equivalent of fast food.
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During the Edo Period, there was a rapid rise in food stalls. Yohei Hanaya (1799 to 1858) is credited with inventing "nigiri-zushi", or the "hand-pressed sushi". (However, it's certainly possible others were also making this type of sushi.)
The sushi of that era was closer to an onigiri with raw fish on it that the smaller pieces of sushi eaten today.
Above, on the left is Edo Period sushi, while its modern era counterpart is on the right.
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According to website Tenpo, the flavour was also different. Today's sushi is made with vinegared rice, but Edo Period sushi was apparently made with rice mixed with red vinegar that been made from sake lees.
That means the rice was reddish brown in colour, and not like the off-white or white rice of today. It was then eaten with soy sauce like now.
— SPPMIZUHO (@SPP_MIZUHO) February 28, 2018
Tenpo adds that two pieces of Edo Period sushi would be enough to stuff one's belly, making these convenient and filling meals to eat on the go.