This really isn't a one-off experience; I've switched back and forth between Diamine and Noodlers several times in some of the pens; that doesn't explain why my pens write 'fatter' if they're filled with Noodlers; and it also doesn't explain why that's my consistent experience with my dip pen, too.It could just as easily be that the fresh change of ink when you retested your pens fixed any of the flow problems from neglect etc though...
It also strikes me as quite unremarkable that different inks might have different properties...