Windows automatically expands the columns as you add items to it. each group contains two columns (which is only one column if all the items are large tiles). When you "spill over" it creates a new 2 column section.
To put this another way. When you create a new column and add a single small tile, there is only one column, if you add a second small tile, it creates two columns. If you add a full-size tile, it is two columns wide. As you add tiles, each is added below the others in the column until it reaches the end, and then it creates a second set of two-column tiles as you continue to add.