Thanks, disco_bob, for your extensive explanation of the situation, which leads me to a few reflexions that I would like to share here.
I can understand the idea behind the space saving device-tailored (trimmed) apps, especially with the 12.9 in iPad in mind. I can't indeed imagine an iPhone 5 having to carry the new iPad's 2732x2048 images along with its own custom 1136x640 images. And the code needed to drive the iPad is obviously much heavier than that driving an iPhone 5.
I am in a location where bandwidth allowance and a slooowww connection is much more of a problem for me than disk space. I can manage the latter but I can't do anything about the former's limitations. So, having to triple-download all my apps (iTunes, iPad, iPhone) will get to be a nightmare. But then, why doesn't Apple give me the choice ? Full, or tailored downloads depending of the individual situation.
A question arises : suppose my iPhone crashes and I then have to go over a complete restore from a backup. Suppose also that its disk is almost full with trimmed apps. Either my iPhone's apps will all have to be re-downloaded afresh, or the full versions of all them stored in my local iTunes will be uploaded to the device. In the first case, it will consume a full two months of my bandwidth. In the other case, the whole purpose of this new app trimming will be defeated, as the full versions of the apps stored in iTunes will be installed on the iPhone. Which will then lead to an obvious overfill.
I venture a suggestion to Apple : why not build devices with expandable disk space, like micro SD cards, for example ?
Then, managing this whole complex situation would never have arisen.
Unless there is something I didn't catch ...
Or unless the forthcoming El Capitan deals more intelligently with it.
YM