Fujifilm X70 has a 16.0MP APS-C (23.6 x 15.6 mm ) sized CMOS X-TRANS II sensor and features EXR Processor II processor. On the other hand, Fujifilm X30 has a 12.0MP 2/3" (8.8 x 6.6 mm ) sized CMOS X-TRANS II sensor and features EXR Processor II processor.
Below you can see the X70 and X30 sensor size comparison.
As seen above, Fujifilm X70 has a 6.3x Larger sensor area than Fujifilm X30. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.