@joaodesousa that’s how Casper’s design works.
On the homepage, the first and every 6th post has the
.post-card-large class which forces a full-width, slightly different design.
In-between the large posts on the homepage, or for all posts on a non-homepage index, each post will fill the available space up to maximum of 3 in a row if you have a wide enough screen.
You can see this in action on the demo site:
Homepage index - shows the first and every 6th post having “large” styling
Tag index - shows every post taking up the available space, note the last post in the index is full width