The iOs developers made a boo boo and set a small maximum font size of emojis.
The problem is apparent on iOs 7 and iOs 8. You can see it while browsing emojier.com, that the icons are really small and useless. It is impossible to scale the font to a larger size than the default text size. So if you set the font size of a text to 40 points the emojis of the text will not scale up.
It seems a bit of a strange decision to limit the emojis font size. A font is suppose to be scalable and Apple should know this and probably they also do know it. Hopefully the reason is that they were lazy and did not have vectorized images and did not want to show badly scaled emojis. But it looks just as bad to have big text with tiny emojis.
Well, not everything is order in the Apple house. Since Steve Jobs left us the Apple has started to rotten a bit. Basically the problem is that it is impossible to scale the emoji fonts above the default font size. Well, we hope Apple realize that this is a mistake and fix it in iOs 9. There is some suggestions to pre-zoom the webpage with the viewport or use absolute positioning tricks. Those are not real solutions, but basically all that is useless. Big boo boo Apple. Fix it now! It does not hurt to go on the Apple forums and complain like hell about this.
Update: On iOS 10 you can set a large size, but the emoji is not a vector image and it gets pixelated!