why are you estimating my bigger house price lower than a smaller house next door?

I would like to know if all your home facts like your addition square footage listed on Zillow is correct. If not, correcting it might increase your Zestimate to a more accurate number. You might also look at the taxes paid on the smaller house. If they are more, their assessment may be higher. A higher assessment can also affect the Zestimate. I can't explain Zestimates, but certain things do influence them. If the buy price for the smaller dumpy house was more than yours due to when it was bought, that would also affect the numbers. If you haven't claimed you house yet, I would so that you can remain private, but so Zillow know which house to look at. Also, you might consider doing an owner's estimate to include additions or remodels that Zillow does not have listed. These can also affect your Zestimate in  positive way once Zillow can see them in the numbers. Zillow is all about numbers, if they are wrong, dated or scewed by a foreclosure or bubble sale that's why they may be off. Personally, I find the Zindex and the recently sold houses to be the best indicators of value available on Zillow.