When I was looking for magnesium a while back, I looked at all the different types and went for chelated. Can't remember the details, but it was to do with absorbability, I think.
So worth checking which type of magnesium is in the tablets.
@Totto your Holland and Barratt link goes to a version with Magnesium oxide in it - you may want to check that is the variety you want.
Guessing here, but it is possible that the size of the tablets is dictated by the variety of magnesium in them... maybe.