AFAIK, widely accepted interpretation of the written rule is that when sorcerer type casters cast metamagic version of a spell which has "1 action", that act becomes a full-round action, but that is different from casting a spell with casting time of "1 round".
When you cast a spell with casting time of "1 round", the spell takes effect just before your next turn. And until that, you are "casting" that spell. Thus, any damaged taken during that period forces you to roll a Concentration check.
On the other hand, if you cast a metamagic version of "1 action" spell, it is just a full-round action. You can only take a 5-foot step and some free actions (or one swift or immediate action). But the spell takes effect on your initial turn.