The penetration ability of a HEAT round, including ATGMs, is proportional to the diameter of the round.
That is true, if the larger diameter warhead is simply a scaled up version of the smaller and uses the same explosive filler, has adjusted standoff, same cone material and shape, etc.
However, there are many counter examples. We can look at the performance of several US and Soviet ATGMs over their life cycles and see improvements in penetration, all the while using the same missile package - diameter, length, sometimes even weight (warhead or overall). TOW-2 vs TOW has 29% more explosive, a 50% increase in warhead weight, 43% increase in penetration. Same charge diameter.
It's more nuanced that just diameter.