Stewer is correct, except in critcical situations.

Morphs just change the existing geometry. The amount of data needed to re-locate a vertex of  the geometry is very small. For single characters, the difference is insignificant. However, if you have lots of characters in your scene, then the bytes can add up.. Probably not worth worrying about tho' unless you a seriously lacking memory.

Textures: A similar sort of thing, but textures (even with maximum texture size reduced in FireFly), still take up the same amount of memory inside Poser. The texture-size reduction takes place at render-time only, so the whole texture is still sitting there in Poser's memory area. And FireFly will be fractionally slower, as it has to do all the calculations to reduce the texture size as it renders.

If you only have a few figures/props, then the difference can probably be measured in milliseconds. But if you have a large number of figures/props (eg, 4 V3's) all with (eg) 4000x4000 textures, then it's better to create reduced size textures outside of Poser and use those instead. Especially for any figures/props in the background.

And once you've reduced your textures, be sure to:
a) Load your PZ3 scene (the one with the big textures).
b) Load and apply any (externally) reduced textures.
c) Re-save the scene (as a PZ3, preferably under a new name)
d) Exit Poser.
e) Restart poser and load the recently created scene.

That will flush any large texture out of memory (Poser will otherwise try to plod on with all those large textures still hogging memory, even though they're no longer used).

Hope that helps
Cheers,
Diolma