Yes, you can easily build the same house on an R5 lot as on a R7 lot. You will have more land on the R7 lot left over to take care of. The issue comes down to density. The smaller the homes and the more homes per lot increases the density. This is what people who have large homes and large lots don't want. They want spacing and less population. The best solution is mixed housing. Do a row of town homes like 1,500 sq ft, some duplexes on the corners, and sprinkle the variety into the neighborhood so it looks nice and still does not overcrowd. If you want to keep your mansion, then keep it.