An image added to the page fills the container that it's dropped into – this is because the layouts change depending on the screen they’re displayed on, so an image doesn't have a set size, it will enlarge or reduce depending on whether the site is being shown on a mobile or desktop - for example an image added to a 3 column layout will be a 1/3rd of the page wide when viewed on a desktop, but will be full width on a mobile.
You may find that some images you have uploaded have different 'aspect ratios', maybe your camera uses a different ratio to your mobile for example. You may want to standardise these so that the images look neat and line up on your web page. You can either do this with an image editing package, or there is a crop tool within the image picker window. If you crop an image within the image picker, it will create a copy with the new cropped image.
If an image is showing too large within a container and it's not possible to control get the desired size using layouts, then we have introduced a ‘maximum width’ setting on the image that will ensure the image doesn't go over the specified size.