Is there a difference in bushing size if the car has a big block vs a small block? Car is mine in the sig. or is there a difference between 73 74 B bodies
I am trying to understand how the bushings could be to small

The suspension changed on the 73 and up b body cars, but the bushings would be the same between big block, and small block.
The car doesn't care what size engine is in it.
Now, bushings for 62-72 b bodies, are the same, but lots different from 73 up cars.
Do you have bushings from 62-72 cars? and then there is your problem.

the shop that has the car just told me the bushings they got match the ones from the broken control arm but are too small for the used one I ordered from desert valley auto parts in Phx.

If the used control arm doesn't match up to the one that came off your car, then the replaement one is obviously wrong.

thanks hemi71x that's what I was thinking too

These are 73 Charger control arms.

I have the "curse of the 73/74 B body" as well

Many places will try to sell you parts that fit '72-earlier B-Body,
and 73/74 A-Body parts because they think they match or a book says they match.

And while much stuff does interchange like that, you will find a ton of stuff that fits an up to 72 Body, and all the 70-74 A bodies, but not a 73/74 B Body.

I believe your UCA and bushings is an example of that.