External Monitor displays un-true 1080p (need to underscan with Yosemite)

[Edan]

Hi, recently upgrading to the new 5k iMac with Yosemite, im now facing an issue with my External Monitor. Previously I have been connecting (via a thunderbolt to hdmi) my Macbook Pro with Mavericks to my 1080p TV and it has been displaying the image perfectly, The screen fits and I never had to underscan the display and the text was nice and clear. Now connecting the TV to the iMac, the image is worse and it doesn't fit the screen properly, having to understand the display has resulted in crunched up text and it doesn't look nice.

I'm not sure what the cause of the issue is, but any feedback which can help resolve this issue is greatly appreciated.

Thanks,
Edan

iMac (Retina 5K, 27-inch, Late 2014), OS X Yosemite (10.10.1)