Hows about an opinion or two.

120/240 volt 1 phase feeder pulled from gang meter to townhouse.......one 2/0 ungrounded and one 3/0 ungrounded conductor along with the neutral and ground.
Should there be concern for the difference in size and is this a violation.
Not sure which way to take it but something does not seem right. Maybe the same logic stated that parallel conductors shall be the same size?

That is a odd one.
First thing I would consider is if the 2/0 is ample size for the load and over current protection.
I really don't know why some one would do this except for they needed 2/0 and ran out so ran the other leg with 3/0.

What does the contractor say?
If you start down the road of voltage drop or adjustment correction factors then it does not hold water. Then you also need to consider the neutral and ground size.

IF IF IF the 2/0 is big enough. AND AND AND the neutral and ground are big enough (see the calculations and tables and charts) THEN having one wire be bigger is just a plus.

Today- it probably means that they had some 3/0 in the scrap bin and did not want to buy more 2/0. Bigger is better. Bigger is allowed. The "unbalance" does not really exist. What if they used both ungrounded conductors as 3/0? or 4/0? or 500 KCMC? jUST BIGGER.
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BUT!

BUTT!

(still have not mastered the "TAB" key and entry key)

I still bet the bigger wire was on hand and used to save money. I would have!

I agree with 32513. Assuming the mains or calculated load does not exceed the 2/0. The same would apply to replacing only one RHW leg with THWN as a cheap repair...This is not the same principle as parallel sets but similar in that each set must be identical but not with another leg of the same run...NEC 310.4(B)