It is to be expected.
The nature of digital data is that there is only so much data there.
For images this means you only have the pixels that are there available. If you scale up these pixels will be scaled as well and it will at a certain level look pixelated.
There are techniques to compensate for some of this, such as interpolation. There are also more recent techniques that is able to do a pretty good job converting it to vectors first (see this link for example).
You can always obscure the pixelation by camouflaging it with something else, but it will still be pixelated or somewhat look altered.
In the end however, these pixels needs to be re-created somehow by actual pixels or the illusion of such, and it's in the hands of the algorithm (and artist) how well the result of this will end up to be.
The better approach is to get hold of a high-resolution image for big prints. The pixelation is a tell-tale sign for that 'this might not work' and 'extra work ahead' :)