The photo did not change size. Image ▸ Canvas Size… only moves the layers; the new canvas area is filled with the Canvas extension color, which defaults to Background — your background color, usually white. Choose Transparent there to add empty canvas instead.
Symptoms
- You open a photo, choose Image ▸ Canvas Size…, make the canvas larger and click OK.
- The photo's layer now has a white or background-colored border, so the layer looks resized or padded.
- The photo content itself keeps its size and position.
Fix
- Choose Image ▸ Canvas Size… again.
- Set Canvas extension color: to Transparent.
- Enter the canvas size you want and click OK.
The new area is now empty. If you also want to change the photo's size, use Image ▸ Image Size…; that scales the image pixels.
Why this happens
An opened JPEG or another fully opaque image becomes a locked Background layer, as in Photoshop. A Background layer cannot contain transparent pixels, so when the canvas grows, Picto fills the new area with the canvas extension color. Background is the default, so the border takes your background color — white until you change it. Layers that are not the Background are only moved and keep transparency.
Canvas Size never scales layer pixels: it moves them by the Anchor: you choose. Scaling is Image ▸ Image Size…'s job.
Still stuck?
If the photo itself is scaled or cropped, or the border appears with Transparent selected, report it in the issue tracker with your version from Help ▸ System Info… and a screenshot of the Canvas Size settings.