For a world with the same mean density as Earth, I calculated a diameter of 127,243.86 km, or 79,065.669 mi, so your T20 World Builder must be presuming a world with approximately the same mean density as Earth.

In *Traveller* terms, 1977 Book 2, p. 26 has planetary formulae, the first two of of which are R = D ⁄ 2 and G = K (R ⁄ 4). Since 1 D = a diameter of 1,000 mi, 1 G = Earth gravity, and 1 K = mean Earth density, combine these two formulae to make G = K (D ⁄ 8), which leads to D = 8 G ⁄ K, or D = 8 × 10 ⁄ 1 (for a world with the same mean density as Earth) = 80 = a diameter of 80,000 mi, or 128,747.52 km. Since D typically comes from the size digit of a UWP, a D of 80 is literally off of the Book 6, p. 26 size table. To get the D of a 10 G world back onto the size table, increase K accordingly.