Why not breed a mini?

[Turtlepete]

Unless you focus on a species that has an exceptionally quick reproduction rate (new generations within 2 years) it wouldn't just be a multi-generational commitment of the tortoise, it would be a multigenerational commitment for an entire family. If you were attempting to produce a mini of a popular tortoise such as a redfoot, the project would outlast the creator and require the commitment of their children, should they have any. Decades would go by before you would see any worthwhile results.

There are a lot of other reasons why this wouldn't work the way it does in snakes. Compare it to the popularly "morphed" ball python. Most of the characteristics that are desired within snake morphs focus around color and pattern, both of which can be more or less distinguished at birth. This makes it easy to pick a few focus specimens from a clutch and sell off the surplus. This wouldn't work when breeding for size, and you would find yourself stuck with quite literally hundreds of specimens before you were even able to distinguish which specimens would be appropriate to reproduce. Not to mention snakes often produce larger clutches, giving more possibility to find the correct fit for the desired characteristics.

 

[WithLisa]

Tortoise eggs have to be about 20 grams or more to be viable.

I've seen much smaller eggs and I know they can be viable too.
Recently I even found pictures of a 3g hatchling, what a tiny little tort!

 

[Kapidolo Farms]

I've seen much smaller eggs and I know they can be viable too.
Recently I even found pictures of a 3g hatchling, what a tiny little tort!

Fair enough. Homopus signatus eggs (smallest species) are in the range of 12 to 15 grams, about the same as Egyptian tortoises. The result a great deal of evolution. I am thinking more about those Bulldogs.

With chelonians bigger eggs within a species provide larger neonates which have a better survival rate. Even if the miniaturization worked it is all counter to the OP's intent, to have a less demanding species (variant). Maybe the idea is a sole pet for companionship with no regard to the effort that may be required to create it?? Who knows? It's an interesting question to contemplate.

I pretty much like them as found in-situ, in my ex-situ. LOL