Homes on large lots are attractive for many reasons; they provide privacy; the ability to create a wonderful outdoor living space with pool, exercise and entertainment areas.  

If the lot is wide, there may be an opportunity for future development - like severing the lot into more than one parcel, or if the lot is long, perhaps some of the 'missing middle' housing - like row or town homes.  

Finally, smaller homes on larger lots are attractive to builders who can use the land to build a larger home.  Builders usually will not pay top dollar for a large lot, but offer land value to sellers.

In Ajax homes on larger lots are usually found in the northern areas which still remain more rural.  Some of these homes may still be on septic or well, and will require hook up to municipal utilities.  

If you're planning to re-develop on a large lot, make sure you check with the Durham region before firming up your plans.