No its not only you. I put on 18" all seasons on it for winter and it just looks sad.
So I bought the Continental Extreme Contact DWS for ALL seasons in 20" and got my Chrome BMW wheels back and it looks SO good.

You are spot on, on this one.

BTW those 18" wheels and tires (Michelin Green X All season are for sale)

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2011 X5M
2006 X5 4.8is
2009 Porsche 911 PDK