I don't think it's an infringement of the SDK confidentiality agreement to state the following:
Code size: The new form factor and speed of the iPad allows for more to be done and therefore more complex programs are likely eventually. That said, increases in code size are unlikely to be particularly noticeable.
Resource size: Apple is actively encouraging developers to create visually rich applications that take full advantage of the iPad's high quality display for the enjoyment of end users. Apple wants the iPad to be a pleasurable experience and has led by example with applications such as iBooks and the calendar.
Now despite this it is still possible to create beautiful applications from a visual perspective without going overboard on the resources but this is where size differences will be most noticeable. It will be down to developers to come up with ways of minimising footprint whilst maximising user experience.
Now to answer your exact question directly. Will iPhone apps that are enhanced to support the iPad be significantly bigger? Well that will depend entirely upon how the developer chooses to 'enhance' their existing app. If the app is primarily based on Cocoa standard interface elements (buttons, sliders, text boxes, etc) then the answer is going to be "not too much". However, if the developer instead realises that their app has the potential to be "so much more" on the iPad and modifies it accordingly then the answer would be "quite likely". How much bigger will depend on the skill of the developer when it comes to balancing "visually rich" against "resource heavy".