Does a bigger spot size like an 18mm penetrate the skin deeper than a 15mm? I’ve heard this but can someone explain how this is so?
Yes.
The Candela hand piece creates a triangle which penetrate deeper with a bigger spot size.
It’s like with a magnifying glass and light above it.
Can you explain how it penetrates deeper? I understand a bigger spot size will bring down more heat to the surface, much like a larger magnifying glass would do with sunlight. However, it seems to me the heat will penetrate deeper into the skin with a smaller spot size.
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See this diagram? In the first one the two sides of the beam meet at certain depth and in the second one the depth is much deeper cause of the bigger surface area.
Sorry about the bad diagram 
Thanks Odi.
There was a time when no question on this forum got answered unless Andrea or I provided the reply. I can’t tell you how we feel about the fact that now days, when we check the forum, most of the time, some reply has been made, and we frequently don’t even have to add anything to what was said… other than a thank you every now and then. 
I’m just glad i can help.
yes, thank you =)
Laser works in an upside down cone. Look it up online. That’s why it penetrates deeper with a larger spot size.