Kublai dramatically expanded the Mongol Empire in China by conquering the Southern Song. He then proceeded to build a Chinese-style bureaucracy and created the Yuan Dynasty in Chinese style. Reading John W. Dardess's article From Mongol Empire to Yuan Dynasty: Changing Forms of Mongolian Rule in Mongolia and Central Asia, one gets the impression that the Mongols Yuan rulers paid great attention to administrative and economic matters in Mongolia. Then again, Dardess has always argued that the Yuan was a centralized state and gives a very romantic view of the later Yuan court. Other historians will be quick to tell you that Kublai divorced himself from Mongolia the moment he chose to set up capital in Beijing, and that the creation of the Lingbei Branch Secretariat was nothing more than an attempt to bring a backwater region into the Mongols' Chinese Empire. Regardless, there is no denying that Kublai's primary focus was in China, thus he is a very influential figure in China.

Genghis Khan is different. Genghis waged wars in all directions, against many different kingdoms and polities. He laid the foundation not only for Mongol rule in China, but also in the Middle East. The Mongol invasions had a profound impact on world history, and that was all because of Genghis, not Kublai.

tl;dr: Genghis focused on the world, Kublai focused primarily on China.

I am not a Mongol scholar (just an enthusiast), but since there are no other comments, I'll give as much of an answer (based on secondary sources) as I can. Jack Weatherford's book Genghis Khan and the Making of the Modern World, among other literature, is a good and accessible book on this if you want to learn more. I can also provide further citations if needed. A note on word choice to prevent confusion - I use Chinghis Khan as the spelling because that is more accurate with how his name would have been pronounced (source for this is just time spent in Mongolia).

Beyond the fact that Chinghis Khan united the Mongol tribes and then established the Mongol Empire (both of which are incredibly impressive feats for reasons I'm happy to explain), there are a few factors at play that explain why the growth of the empire under Chinghis Khan is fundamentally more impressive than under Kublai Khan's.

I think the answer to your question lies first in a fact you cited that is incorrect. Kublai Khan did not take power in 1227 (the year of Chinghis Khan's death) - he took power in 1260. After 1260, the Mongol Empire did not undergo the significant growth you're describing, although its borders certainly did continue to expand, particularly in Russia and China. As a result, the level of growth you're assuming happened under Kublai Khan is overstated.

Another dimension that is important is the structure of rule. While Chinghis Khan and Kublai Khan were both the top leader (or great Khan) of the Mongol Empire at different points in time, Kublai Khan's rule saw an entirely different structure, one that drastically decreases the direct effect Kublai Khan had on the growth of the empire as a whole. This structure was the break up of the Mongol Empire into 4 khanates - Batu's golden horde in Russia, Chagatai's rule in Central Asia, Hulagu's Ilkhanate, and of course, Kublai Khan's own Yuan Dynasty in China. So, Kublai Khan only had primary control over China. While the area controlled by the Mongol empire in China expanded significantly under Kublai Khan, this growth is less impressive than the immense growth of the Mongol Empire under Chinghis Khan into China, Central Asia, the Middle East, etc. - growth that happened in the incredibly short span of 21 years (since Chinghis Khan officially united the Mongols and was "crowned" in 1206).

Hope this helps! Let me know if more citations or clarity is needed.