Since painting was considered a job, Giovanni Paolo Panini had his "factory" set to produce as many paintings as would be requested, a full production chain, and a few image types no doubt were more successful.
Panini and his studio were tendentially working on big paintings, 2x2 meters of size, which would fill up a wall in Castles and rich houses.
On the other hand, Canaletto was requested to work on smaller formats by his Patrons: Owen Swiny or Joseph Smith would usually limit his size choices in no more than 1x2 meters and even less, 70x100 cm sometimes, as they were easier to transport from Venice (my assumption) and maybe they were less expensive? or faster to make? (detailism in Canaletto's works is quite impressive). A big postcard, anyway.
Very often Panini's serial paintings are so alike one from the other, that hard work is required to find the differences from one painting to the other, as we can see in these examples here:
We can say that mass production of paintings was going on in Panini's studio, starting from around 1730.
Nothing strange, but simply a trend which could have been determined by the factor of Rome being such an important (and required) tourist destination, and the desire of these rich tourists to "take home a souvenir" to share memories and sensations with family and friends.
And a 4 square meter of beautiful painting, sure did the work.
And tastes in acquiring Art were changing in those times.
Holland was in the process of moving out from Catholicism to enter the Protestant Religion, and the artistic tendency had changed already because of a change in "trends" or "tastes" of who could afford to buy paintings to decorate their homes.
Food Art and Genre Painting had already been trendy for a while (end of XVI - mid XVII century):
Also Johannes Vermeer, in mid XVII century, who had started his career as a professional painter with Religion inspired paintings:
... had moved to "Genre Painting" in his quest for recognition & success ...