Don't jump to conclusions - it could be lateral variation
We have moved sideways as well as generally up-section. Unless you can corroborate a similar size variation with height at one location, it is actually quite possible that lateral variation is involved.
On Earth, you'd do longer traverses and cross-traverses and drill vertical holes to prove the 3D occurrence. On Mars we're limited by the lifetime of the rovers (or their funding for support operations) and we may never have a definitive answer.
On Mars, it pays NOT to make your mind up until you've tried looking from several different angles and allowed for the limited dataset.