you doing great well done just a few notice here :
the scope of Gas giants is rowgroup
you forget the scope for Dwarf planets which is row
now for your questions
if you mean that the cell is smaller i inspected one of the cell and they has the same size
colgroup is used to target that group with css or js so maybe you want the first 3 coloumn to has some color and the last 7 to have different color
so you group them then you target them to do what ever you like