Panasonic ZS15 has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CMOS sensor . On the other hand, Panasonic ZS25 has a
16.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CMOS sensor .
Below you can see the ZS15 and ZS25 sensor size comparison.
Sensor Size and Resolution Comparison image of Panasonic ZS15 and Panasonic ZS25 Cameras
Panasonic ZS15 and Panasonic ZS25 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Panasonic ZS15
or the
Panasonic ZS25 ,
including hyperfocal distance.