Could the multiplication of matrix X (with dimensions [d+1 x N]) and its transpose simplify to a matrix with [d+1 x d+1] dimensions?

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In a machine learning course I'm taking, one of the lectures deals with matrix multiplication.

In particular, the following was presented: Snapshot of part of slide

Could anyone explain why the dot product of these two matrices would "shrink" to [d+1 x d+1] dimensions instead of being [N x N] dimensions? Thank you.