What this video covers
The clip takes on the assumption that a small visible patch means a small problem.
What is on the surface is the part of a colony that reached daylight. Behind it, growth has usually spread through the material for some distance, which is why a patch the size of a hand can turn into a wall section once anyone opens it up.
The takeaway
Size on the surface is a poor guide to size overall. Treat a small patch as a prompt to look behind the surface rather than as reassurance about scale.
Read the full written guide: Why a Small Mold Spot Is a Big Problem.