Saving PDF in Photoshop

Impossible to know without any info about the axctual specs of the file, the type of content and yes, ultimately what PDF settings were used. The most likely cause for file size changes are things like text objects that meay need to be rasterized as images because of effects applied or operations affecting an image's coloring or how it interacts with other objects. This works in both directions, as obviously files can also get lighter when you e.g. substitute graphical text with editable text based on embedded fonts. Similarly, PDF presets can override soem of these options and there's a huge difference in file sizes. based on your description it could well be that you have an image that was previously not changed and embedded fully as a TIFF, but now you have doen something to it that causes it to be converted and resampled to a smaller JPEG. Anyway, really impossible to know without specific info.

Mylenium