Tom,
I set it as large because if I set it as small I have only one column on the left with all small posts one below other, and its looks bad.
Before the problem, I was setting it as one large post (the latest one) and below small post and I was having 1 large post and below 3 columns (Small Post 1 next to Small post 2 next to Small post 3). I don´t touch nothing which could explain why the setting of the masonry changed.
Regards.