Why is the picture so shrink and is there way to make that image full size like snapchat ?

Do you want the image to be the size of your view or a fullscreen-activity, too ? For a fullscreen-activity, use the following in your styles.xml:

        <style name="<your name>" parent="android:Theme.Material.NoActionBar.Fullscreen"/>

For displaying your image on the whole screen, make sure your layout and imageView has no paddings/margins at all and width/height set to match_parent.

Picasso gives you the option to resize the loaded image to suit your needs. For Example:

       Picasso.with(context).load(Url).resize(width, height).into(ImageView);

Note that Picasso takes actual pixel-values here, not dp-ones.

To get your displays dimensions you can use Androids WindowManager (when in an Activity)

    Display display = getWindowManager().getDefaultDisplay();
    Point screenSize = new Point();
    display.getSize(screenSize);  // the values are saved in the screenSize object
    int width = screenSize.x;
    int height = screenSize.y;

For further informations, take a look at this post

I hope this works for you.

Best regards, Fabian