Even though the Weasleys looks like a huge family, they’re actually a shadow of their former terrifying selves. In the past there were so many Weasleys their family dominated Britain.
The weasleys in the past numbered in their THOUSANDS. Weasleys infested every level of government, every shopping street, every school. Half the aurors muttered fearfully about how the weasleys could overthrow them easily, although they made sure to mutter this away from the other half who were also Weasleys. There were so many weasleys they were considered a legal minority, politicians who ran for office (the non-Weasley ones) had to run on pro-Weasley policies for any hope of election and making an enemy of a influential weasley was often considered career or social suicide, although the silver lining was that were were so many Weasleys you could often regain social status by just befriending another weasley.
The reason for everyone ‘looking down’ on the weasleys isn’t snobbishness, its an old survival tactic. Every rich family in the past who wed so much as a single weasley immediately found itself inheriting several dozen relatives to which they had to at least provide sinecure positions. Any family, no matter tis size, was immediately a Weasley puppet the moment they married. Fondness and the urge to marry the weasleys must be erased from any noble before any of them has any funny ideas.
Molly in the past would actually have been considered average. Seven kids? Not bad, although Molly actually views herself as an underachiever, her grandmother was just one of twelve and her mother one of ten.
The reason Grindelwald gained power so quickly was because he was the first politician in living memory who was not only anti-Weasley, but had enough power to survive the Weasley’s first public attempts to remove him. In a flash he drew the support of a dozen pureblood noble families and a hundred other smaller organizations who wanted the Weasleys suppressed. Muggles? Who cares about that, we can worry about that when there aren’t two separate address books, one for the Weasleys and one for everyone else. Better (or worse for the Weasleys) they were beginning to fracture into one of their usual destructive Weasley civil wars (the only reason they haven’t completely infested the entire planet) This time was over pro and anti-Muggle camps, and Grindelwald took advantage of this Weasley civil war to annihilate them.
The war between Dumbledore and Grindlewald was utterly destructive (started in this universe because Dumbledore, watching Grindelwald massacring Weasleys, decided to step in). By the end of it, most of the Weasleys were either killed, scattered throughout the world, or renamed themselves to shed the social taboo. The only family left in England who openly held the name was Arthur and Molly’s family, and even then they only survived under Dumbledore’s personal protection.