Maybe it's for the plot. All of Ianto's nicely tailored suits got blowed up (you see, he had been to the dry cleaners that day and naturally brought all of his suits in with him on his way to work) and he's forced to wear his "fat clothes". It's possible. Or he's had to run way too much (as evidenced by many outside shoots) and during the course of the five episodes, you can see him practically waste away...

Or the BBC's just too cheap to take in a few spots here and there.