Few problems, I believe.
As pointed out already, 225 is a standard size breaker. In your 1st example your primary conductor would need to be a 4/0 IF you use the 225 amp breaker.
Also, on your 1st example, if you are going to use a 300 amp OCP on your secondary, 240.21(C) prohibits the "next size up" rule, so your conductor(s) would have to have a rating of at least 300 amps.
On your 2nd example, I am of the opinion that you can not use a #2, with a 115 amp rating, on your 125 amp breaker. Although the Code allows the "next size up" the conductor must be sized for the load and there is nothing to prevent the lad reaching 125 amps.