Step 1: Set up Euclidean division.
Let the smaller number be $s$ and the larger be $L$.
“Quotient $6$, remainder $15$” ⇒ $L = 6s + 15$.
Step 2: Use the difference condition.
$L - s = 1365 ⇒ (6s + 15) - s = 1365$
$⇒ 5s + 15 = 1365 ⇒ 5s = 1350 ⇒ s = 270$.
\[
\boxed{270}
\]