[SOLVED] Images in Pages not Original Size

I'm pretty certain I'm doing something stupid here but would appreciate somebody telling me what !

I am inserting web optimized JPG images which are 400px wide into my page but they are showing in the page as 300px wide only. If I click on the images, they scale up to the 400px width but why are they scaled down in the page itself ?

The image attributes state 100% and when I click on "original size" they show as 300px but they are definitely 400px because I've downloaded them and checked in Photoshop.

Is there a control somewhere that shrinks them down to a default maximum width when uploading or inserting into a page ?