I will break this down as simply as I can (and list the numbers at the bottom):
The first hammer breaks HALFWAY through ONE block. That's 1 block every two taps. The second one breaks FULLY through ONE block. That's 2 blocks every two taps. Good so far. The third one breaks HALFWAY through TWO blocks. That's... 2 blocks every two taps again. Wasting money on an upgrade that does nothing. The fourth upgrade breaks FULLY through TWO blocks. That's 4 blocks every two taps. Back on the right track. The fifth breaks HALFWAY through THREE blocks. That's 3 blocks every two taps. That's WORSE than the last one. This pattern continues (whole 3, half 4, whole 4, half 5, whole 5) and the gap gets worse and worse between the halves and the wholes. And keep in mind EACH UPGRADE IS MORE EXPENSIVE THAN THE LAST. YOU ARE PAYING MORE FOR A WORSE ITEM.
From the start, here is how many blocks you get for every TWO TAPS for each hammer (obviously you want this number to keep going UP and only half of them do that): 1, 2, 2, 4, 3, 6, 4, 8, 5, 10, 6, 12, 7, 14, 8, 16, 9, 18, 10, 20 (and so on)