Not true in my experience, though i don't usually go for a small breast build. Bigger breasts only happen when using the cream over multiple days, or after a dream. AFAIK using the cream adds to a growth variable, which doesn't reduce over time. So if you use the cream X times, you're guaranteed breast growth no matter how much time between uses.
The traits from the bimbo magazine are random, you can get them after reading 1 time or it could take dozens of times (easily tested by using a quicksave to save scum it).