Size and weight is a big decision factor when you are trying to find the ideal camera for your needs.
In this section, We are going to illustrate Panasonic ZS3 and Panasonic G2 side-by-side from the front, back and top in their relative dimensions. Panasonic ZS3 has external dimensions of 103 x 60 x 33 mm (4.06 x 2.36 x 1.3″) and weighs 229 g (0.50 lb / 8.08 oz) (including batteries). Panasonic G2 has external dimensions of 124 x 84 x 74 mm (4.88 x 3.31 x 2.91″) and weighs 428 g (0.94 lb / 15.10 oz) (including batteries).
Below you can see the front view size comparison of Panasonic ZS3 and Panasonic G2. Panasonic ZS3 is clearly the smaller of the two cameras. Its body is 21mm narrower, 24mm shorter and 41mm thinner than Panasonic G2. As you can tell, actually this is not a fair size comparison since Panasonic ZS3 has a built-in lens but Panasonic G2 doesn't. Once you put a lens on G2, its thickness will increase significantly.
Now lets look at the top view comparison of Panasonic ZS3 and Panasonic G2.
Weight is another important factor especially when deciding on a camera that you want to carry with you all day.
Panasonic ZS3 is significantly lighter (199g ) than the Panasonic G2 which may become a big advantage especially on long walking trips.
Again, this comparison may not be very useful in real life as we are comparing the weight of a camera with a built-in lens to the body-only weight of an interchangeable lens camera. Once you put a lens on G2, the total kit will weigh much higher.
Unfortunately neither Panasonic Lumix DMC-ZS3 (Lumix DMC-TZ7) nor Panasonic Lumix DMC-G2 provides any type of weather sealing in their body, so you have to give extra care especially when you are shooting outdoors. If weather sealing is a must-have feature for you, check these sections: Top Compact Cameras with Weather Sealing Top Mirrorless Cameras with Weather Sealing
LCD Screen Size and Features
Panasonic ZS3 and Panasonic G2's LCD screens has the same diagonal size of 3".
On the other hand, Panasonic G2 sports a Fully Articulated screen
which you can change the angle and position of the screen almost freely, making it possible to shoot from waist or over the head levels, and especially very useful for shooting selfies and video. Panasonic ZS3's LCD screen is a fixed type which provides almost no flexibility in shooting positions compared to G2.