No. You are really struggling with this.
If it's "no bigger than ours" it may be the same size or it may be smaller.
If it's "no smaller than ours" it may the same size or it may be bigger.
If you say "it's no bigger and no smaller than ours" it means it is exactly the same size.
You say "no bigger than" something you see as relatively small.
You say "no smaller than" something you say as relatively large.
The point is to set up a contrast of expectations.
If you think your house is small, and you are surprised to find theirs is small, you say "no larger than ours."
If you had expected theirs to be very small, then you compare it to your house, which you had thought would be larger relative to theirs: It was no smaller than ours.
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