From the "Combining Magical Effects" section of the Player's Handbook (p. 205) and the Basic Rules (p. 85):

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.

This rule can lead to one of two different rulings:

1. The DM could rule that, since Enlarge/Reduce is a single spell, the more recent casting of it (Reducto's) would take precedence, and Inflatoman would not only return to normal size but actually become Small.
2. The DM could rule that Enlarge and Reduce — while the same spell — are considered separate for the purpose of combining effects, and rule that the two effects cancel out, resulting in a Medium-sized Reducto.

If Engorgo were to stop concentrating, then Inflatoman would either remain Small (option 1) or become Small (option 2). If Reducto were to stop concentrating, then Inflatoman would become Large in either case.

Thank you to @V2Blast for notifying me of the 2018 PHB errata on this!

From the description of Expansion:

Multiple effects that increase size do not stack, which means (among other things) that you can’t use a second manifestation of this power to further expand yourself.

So, the answer is no.

As mentioned by @HeyICanChan, it is also important to note that in D&D 3.5 the Psionic Powers come with the Psionics-Magic Transparency rule. And therefore:

Multiple magical effects that increase size do not stack.

Applies to Expansion as well.