Andrey, see this link: https://stackoverflow.com/a/25818846/966789
(2014) You cannot choose a new size for the image; it will only look good if you keep it at the 44x44 size. The reason is that full vector support is not implemented ... Why it (probably) works this way: (1) This makes it backwards compatible with previous iOS versions. (2) Resizing vectors may be a computational intensive task at runtime; by implementing it this way, there are no performance hits.
This is discussed in 2014 WWDC Session 411 - "What's New in Interface Builder" at time 44:13. Note that they say the rasterization is done at build time. Also interestingly on the MAC it will use vectors at runtime. At some distant point in the future hopefully iOS will too.