I cannot replicate this. Setting dotSize=100 does increase the size here compared to dotSize=20 in Builder. Perhaps to debug whether this originates in Builder or in the underlying python libraries, could you try and run the below in Coder? It should present you first with one second of smaller dots, then 1 second of larger dots. And what version of psychopy are you running?
from psychopy import visual, core
win = visual.Window()
stim = visual.DotStim(win, dotSize=20, nDots=20)