People really have to understand the nature of the Adeptus Astartes. They are an ultra-elite fighting force. They do not (for the most part) fight wars of attrition, they tackle the toughest battlefield objectives often in support of the Imperial Guard.
Gone are the days when Legion sized forces battled the enemies of the Imperium as a frontline force. The Adeptus Astartes now usually act as a rapid-reaction force, deploying quickly and cleanly to an area in support of PDF or Guard forces. Chapter's don't have the numbers, resources or supplies to fight prolonged wars by themselves.
Increasing the chapters sizes to 2,000 Astartes would be pointless and actually counter-productive. It wouldn't be possible to just suddenly recruit an extra thousand Astartes into each chapter which means the numbers would have to come from existing chapters. Which would mean cutting the number of chapters roughly in half, which in itself threatens to spread them too thinnly across the massive space they have to defend. The formation of the Adeptus Astartes from the Legio Astartes was a sensible move given the nature of the Imperium and how it needed to be defended, as well as not allowing individuals too much power and influence over the elite forces of the Imperium.
the pre heresy ultramarines had > 100 K marines, that means there are recruit supplies to muster this number (and that is before all those planets were re-united), now my question is, do the the successor chapter add up to this number ? my gut feeling says no.
The Ultramarines Legion numbered ~250,000 Astartes Pre-Heresy. Considering the Ultramarines Legion is ultimately responsible for around 60% (is that right Baron?) of the current Astartes Chapters, that adds up to about 600,000 Astartes drawn from Guilliman's geneseed.
As for the overall number of Astartes, there are ~1,000,000 Astartes in M41. Pre-Heresy (going by the average Legion size being 100,000) there were ~2,000,000 Astartes prior to the Heresy, probably slightly less - considering how much under-average some Legions were in terms of numbers (Emperor's Children and Thousand Sons as examples). So we can say there are roughly half the number of Astartes in M41 as there were 10,000 years ago at the height of the Great Crusade.
This is an acceptable decrease in numbers considering the Horus Heresy, subsequent Scouring and disappearance of the Primarchs. The Legion's suffered massive casualties during the Heresy and Scouring, and the disappearance of the Primarchs made the process of creating Astartes much more difficult. Add to all that 10,000 years of constant warfare and the number of ~1,000,000 Astartes is acceptable.
However the Ultramarines seem to be the only Legion that have increased in Astartes, at least in the sense of total number of Astartes carrying Guilliman's geneseed. This can be put down to Guilliman's influence himself, and the fact that most chapters use Guilliman's template and geneseed for their own chapter. There is also the rumours of the II and XI Legions (from The First Heretic
) to take into account here, which if true further justify this increase in numbers.