Bizarre Brake Issue

I have been servicing my own brakes for years, and I have never run into a situation like this. Bought OEM rotors and Bosch brake pads (on the recommendation of a mechanic friend) and set to work on Saturday.
Got the driver's side disassembled and began re-assembly and that's when I noticed a big problem. The new rotor was obviously larger than the one I removed. The new rotor is 13.3" (338mm) and the removed rotor is 12.55" (319mm). Long story short, after some rigorous research, the new rotor is correct for the 4Runner, and the removed rotor is the same size as a similar year Tacoma. This would mean that the previous owner installed Tacoma rotors AND Tacoma calipers. It also looks like the dust shields are Tacoma sized, as well.
I have no idea why someone would put smaller brakes on a car. Bigger brakes, sure, but never smaller.
Now the questions that I am posing to the group:
1) Why would someone do something like this?
2) Does anyone know what all I am going to have to buy to put it back to factory? Will it just need new calipers, or would the steering knuckle need to be changed as well?
The pictures are of the rotors and the passenger side showing the old parts.
ANY help would be appreciated.

 

Only reason I can think of that someone would do this is to fit 16" wheels on, like the Tacoma or FJ Cruiser can accommodate. The would allow more sidewall in a tire for off roading.

 

That is bizarre. Maybe the previous owner wanted to run 16" wheels, or had to do a collision repair and got Tacoma front end parts from a local salvage yard. They may be 4th gen 4Runner brake parts as well.

2016 mid-year, Toyota switched from M12 caliper mounting bolts to M14 on the 4Runner. Tacoma calipers use M12 bolts, so the previous owner either drilled out the Tacoma calipers or installed Tacoma knuckles. Easy way is to check the caliper bolt head socket size to see which one you got, as it will determine which replacement calipers to buy. 17mm head -> M12. 19mm head -> M14.

You will need calipers, dust shields, that's about it. The little hard line from the knuckle to the caliper can be retained if in good condition. Otherwise, they are pretty cheap at the Toyota dealer, P/N 47314-60311, 47317-60131.

 

I ran Bosch pads for years on my Honda Pilot. Excellent performing brake pad

 

100% agree with this. The bright side your wheel options have opened up quite a bit.

 

Weird, I’ve wondered about what it would take to fit 16 in wheels on the 4runner. I recall having this conversation on trail ride camping trips, but came to the conclusion that it wouldn't be worth it due to the complication with the brakes. But apparently it’s not actually that hard to do lol.

Just curious, did you feel you had adequate braking with the tacoma brakes?

If you’re returning the brakes to stock, make sure you have 17in wheels.

 

^Stupid thing is pedal feel will be better due to the Tacoma caliper's smaller pistons. However, braking power will be reduced.

Good off-road tires have more choices and better in-stock status in 17" wheel size than 16" wheel size. For example, Duratrac RT is not made in 255/85-16. Cooper STMaxx is usually special order in 255/85-16, while 255/80-17 is in-stock and cheaper. Tire mfrs produce tires to demand, and we're in an age where majority of Jeep Wranglers come with 18" wheels.

 

Photos of part numbers?

Maybe you bought the wrong parts?

 

Thanks for the input guys! Thanks for the parts list! As I recall, the bolts are 19mm.
To answer a few of the questions, I can't see any frame damage anywhere, so I'm fairly confident that it wasn't wrecked. I had a pre-purchase inspection performed, as well.
The 16" wheels theory makes sense, but it had an el cheapo spacer lift installed, so I'm guessing that the change wouldn't have been for better off-roading.
I finished the job after buying new rotors for a Tacoma. I guess having never driven a 4Runner with the factory brakes, I probably wouldn't notice a difference. I think they work fine. They're considerably better than the brakes that I replaced. Probably because of the pads?
I have the stock TRD ORP wheels, so they're 17".

 

I’m looking to get new rotors and pads soon too. How do you guys feel about Akebono? Or stick with OEM? I’m not sure which is best or if that is OEM for the 4Runner. Or Bosch like OP said

 

Stock SR5 with 16" wheels and 319 mm front rotors and pads from the dealer (Advics). Plenty of braking power, like a tractor beam.

 

What I use:

Front:
Rotors: Raybestos 980784PER
Pads: Advics AD0976

Rear:
Bosch BC606 ceramic pads
Raybestos 980780PER slotted rotor.