Re: if there was no preview, the bottom would be based on the picture size
This would add some amount of blank space too, in the preview area.
Basically this comes down to:
I don't think there should be empty wasted space (my examples above with "3 line preview" and only "1 line" or none of actual text), same thing with the specific case of "larger image, no preview -- have to make the item taller, unused empty space again".
And with that, sorry, there are no guarantees that a larger (even larger than with "indent previews") image would fit.