In the EPUB - Reflowable Export Options dialog go to the Object pane (photo below) and change the CSS Size dropdown field. That's what controls the sizes for the images overall. You typically want Relative to Text Flow, which will take the image and scale it up or down relative to the width of the text as viewed in the ebook. Note that it's all relative. If you have an image that, in InDesign, is half the width of the page, Relative to Text Flow will make it half the width of the ebook page.
If you see the same results after changing that setting, the individual images might have their own export settings. If you right-click on an image choose Object Export Options, on the EPUB and HTML pane you'll see similar sizing controls. You can change them there, or, back in the above dialog, enable the checkbox beside Ignore Object Export Settings.
Thanks Yves for mentioning the Size options of Object Export options.
Yes option Size in Object--> Object export Options-->EPub & HTML -->Size is used to govern the size of the object to be exported in epub.
You have multiple options to play with there, like fixed will keep the size same as InDesign Layout.
Relative to text flow will relatively grow the size as the display screen increases or decreases.
Relative to text size will emit the values in ems,or you can enter your custom width or height in last two options.
Someone sent me here because I'm having the same problem, and I did what Apel Yves said but I'm still getting images filling only like... 75% of the screen on the iPad (portrait orientation), and the only way for me to see them full size is to double tap on them. It's the first time I'm doing this so I'm wondering: is it even possible to get images displaying full screen automatically in an ePub? Or is the double tapping inevitable? Because it's clearly not working for me, and my images have the right resolution (I've tried them all anyway). Any help greatly appreciated!
The setting mentioned above tells the reader to display the image at full width of the available space. But some readers such as iBooks add an additional margin to this space so the image might only take about 75% of the screen in the default view. In current or previous versions of iBooks, you can't control this. Maybe in a future version.
I'm currently wrestling with this problem. Images in an epub book display fine in iBooks. They look awful In Adobe Digital Editions, typically very tall and thin. Unfortunately, to pass muster as Smashwords, those images must check out in ADE. And the only scheme I've found that works with ADE is setting the size to Fixed, which make the image much smaller than they need to be. None of the other size options work.
Why does this problem even exist? Perhaps 99% of the time we want ID to write out images that always display proportionally. We should not have to jump through hoops, trying this and then trying that to get that result. The epub code that ID writes should always force height and width to be proportional unless we do something to tell it otherwise.
At the very least, epub export options should give us a check box that says, "Keep height and width proportional in resizing."