Something is fake in these stats. 26% of GDP is about the size of federal budget ONLY.
US GDP is about 11 to 12 trillion. Federal budget is about 25% of it. But there are other layers of government and taxes in America.
That organization has been discredited for lack of a comprehensive approach. The taxes they include are not even near all taxes collected or embedded into pricing.
Also the definition of GDP and the measures of GDP are not comparable.
For a definitive assessment of the miserable US Tax System, look at the Report of the Tax Reform Commission. Although they do not compare with other countries, they lay out clearly where all the taxes are and the monstrous effective rates of all taxes combined.