Increasing the size of a warehouse does not always improve data loading performance. Data loading performance is influenced more by the number of files being loaded (and the size of each file) than the size of the warehouse.
Tip
Unless you are bulk loading a large number of files concurrently (i.e. hundreds or thousands of files), a smaller warehouse (Small, Medium, Large) is generally sufficient. Using a larger warehouse (X-Large, 2X-Large, etc.) will consume more credits and may not result in any performance increase.
For more data loading tips and guidelines, see Data Loading Considerations.