The fact cited in the wiki is about the land surface of Trantor, not the overall surface area of the planet. The relevant quotes from Foundation give us the land area:
Its urbanization, progressing steadily, had finally reached the ultimate. All the land surface of Trantor, 75,000,000 square miles in extent, was a single city. The population, at its height, was well in excess of forty billions.
Foundation, Chapter 3, ENCYCLOPEDIA GALACTICA
But Trantor is not all land; it has oceans as well:
Trantor is tunneled over a mile down. It's like an iceberg. Nine-tenths of
it is out of sight. It even works itself out a few miles into the sub-ocean
soil at the shorelines.
Foundation, Chapter 3
Without knowing the fraction of Trantor's surface covered by oceans, we can only compare Trantor to Earth's land area, which is approximately 149 million km2. (148 940 000 km2 per Wikipedia.)
The given 75 million square miles land area for Trantor is about 194 million km2 (to the same amount of precision). So the relative amount of land area is 194 / 149 = 1.30, or 130%, just as stated. Note that this is not a comparison of the relative sizes of the planets, since we have no idea if Trantor is only 20% ocean, which would give it a total surface area less than half of Earth, or if it's 80% ocean, giving it nearly twice Earth's surface area.