Canon ELPH 340 HS has a 16.0MP 1/2.3" (6.17 x 4.55 mm ) sized CMOS sensor and features DIGIC 4+ processor. On the other hand, Canon ELPH 330 HS has a 12.0MP 1/2.3" (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features DIGIC 5 processor.
Below you can see the ELPH 340 HS and ELPH 330 HS sensor size comparison.
Canon ELPH 340 HS and Canon ELPH 330 HS have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Canon ELPH 330 HS has 32% larger pixel area (2.34µm2 vs 1.76µm2) compared to Canon ELPH 340 HS, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.