so there are questions about how you're porting it over. using crosswalk I assume for android? I've run into a similar problem. Normally Letterbox scale works great, but not when brought over to iOS/Android..
The (yes not ideal for all games) solution I have found is to use scale inner.. this fills up the screen vertically but it also pushes the game wider as well so you can potentially lose a significant amount of screen space. The worse case though is going from 16:9 ratio to iPad.. because you're going from widescreen to traditional 4:3.
look at this image.. http://www.rtings.com/images/black-bars ... mawide.png
the bottom row / middle image is a quick example of how much space would get cut off in that extreme case.. but if you're just going from subtle 16:9 to 16:10 or other similar but not identical ratios.. then you can just be sure that you move your game elements / UI in a little bit so less wide screens don't cut off things.
Also if you do scale inner, be sure you lock it to landscape.. because it will REALLY look bad in portrait if you don't.
Alternately if you're making a portrait oriented game you'd use scale outer which would do the same thing but fill to left to right and also have the same cropping problem if you're not careful..
really though the only devices you'd have any real issue with are screens like iPhone 4s and older and iPads. Even iPhones are not that bad.. they are not 4:3 which is what the iPad which is by far the worsts I've encountered..