What you are feeling is called a hot spot. It is because of the small contact area of the pedals as you surmise. It does not affect all people equally but is the reason that outer cages around road pedals are common.
My own preference is to use the A520. I use them both on my road bike and on my MTB even though they are single sided. The cage distributes the load on the feet nicely. If you have gotten used to the 505/515 engagement then you are going to be in for a plesant surprise at the smooth feel of the new generation of SPD. (M520, M540 or A520). Remember the new gen pedals require the new generation of cleats 50 to 51 and 55 to 56.