Understand to make a small tracker is not the problem, the logic circuit and the antenna can be very tiny, in a button embed.
The problem, as always in the battery.
GPS consumes a decent energy so the smaller the tracker, the less it works.
Appropriately, therefore, very little of the trackers and not - who needs a tracker running for half an hour?
Or rather they are, but it's highly specialized products, retail not coming.