Dimensional quality control is critical for wider adoption of Additive Manufacturing (AM) as a direct manufacturing technology. Due to the process' complex physics, AM-fabricated parts still require post-processing with machine tools, which significantly negates its time and cost benefits. In this paper, we investigate product shape deviation for Mask Image Projection Stereolithography (MIP-SLA) - one of the earliest commercialized AM techniques. By studying part fabrication mechanisms, we consider (i) over or under exposure, (ii) light blurring and (iii) phase change induced shrinkage or expansion as the most significant sources for shape deviations. Accordingly, the shape deviation modeling is established to quantify the effects of those influential factors and to understand the deviation mechanisms. Cylinders and cubes of various sizes were built to test our approach. Accurate prediction of shape deviation for all parts serves as a further confirmation of our model.