The current USDM C-Class & Int'l C-Class (both the same size) are more-or-less similar in size/space as the forthcoming USDM CLA & Int'l CLA (both the same size; '14+).
The upcoming redesigned USDM C-Class & Int'l C-Class (both the same size; 4th gen, '15+) will be comparatively larger in size/space as the forthcoming USDM CLA & Int'l CLA (both the same size; '14+).
The CLA is based off the subcompact A-Class platform (which also is the basis for the B-Class). For the CLA, it's stretched to compact-sized market segment.