Strangely, it seems that matlab's builtin function imresize does not reproduce the usual bilinear interpolation.
Let's consider a toy 4x4 image:
F = matrixnormalize(magic(4));
BF = imresize(F,[256 256],'bilinear');
Compare that to explicitly computing the bilinear interpolation:
[X,Y] = meshgrid(1:size(F,2),1:size(F,1));
[BX,BY] = meshgrid(linspace(1,size(F,2),size(BF,2)),linspace(1,size(F,1),size(BF,1)));
BF2 = interp2(X,Y,F,BX,BY,'bilinear');
The difference is obvious in the corners and along the edges. The imresize result has some flat (piecewise constant) patches). Why? It actually looks as if the image was padded by repeated values before upsampling.