So Rule <SG 2> Says that

Per <G 3> , at the beginning of a Match, each Robot must be smaller than a volume of 18” (457.2 mm) long by 18” (457.2 mm) wide by 18” (457.2 mm) tall. Once the Match begins, Robots may expand, but no horizontal dimension can exceed 36”

(914.4 mm) at any point during the Match.

So the way I thought about this at first was that the robot could expand from an 18" square into an 36" square, horizontally, as seen below.

However, The line that confuses me is when it says that

no horizontal dimension can exceed 36” (914.4 mm) at any point during the Match.

As the diagonal of the 36" square is about 51", it is much greater that 36". In that case, then, the robot greatest that the robot could expand to is a circle with a diameter of 36 inches.

That means that the greatest size square a robot could expand to is about 25.5

inches:

Can I get some clarification on whether this is the case, of if I am just overthinking it?