If you go by theoreticians on OTL atomic research was originally split in German, US, British and Danish "programmes." Some of the former and all of the rest eventually ended up in the Manhatten Project. On the TL-191 all three/four plus the CSA programme all remained seperate. (We are ignoring possible espionage and collaberation between the USA and Germany). You lose opportunities for more cross-fertilisation. of ideas with what are secret projects. Ideas are not being bounced off like minds leading to more ideas.
Doesn't that cut both ways though? Yes, having one big centralized project can make it easier for cross-fertilisation of ideas, but it can also produce an environment where group think sets in and people are afraid to voice alternate ideas that go against the group consensus, and the entire project can also be derailed because of bad leadership or a toxic work environment.
Also, it's not as though there wasn't transnational sharing of ideas in TL-191. Professor Fitzbelmont himself mentions that he realized a nuclear bomb was possible after reading about the results of nuclear research being conducted in Germany, and he also notes that once the war starts the US scientific journals suddenly stop mentioning uranium (which means they were mentioning it before the war), so at least until the Second Great War started nuclear research was being freely discussed among the global scientific community in TL-191.
There is also the economic factor. OTL WW2 USA had a larger economy than any TL_-91 country and had not been invaded by another power. Thus it had more resources in total than any of them. Therefore it can afford more on technology whilst putting proportionally the same number of armies, fleet and air forces in the field. It decided that it could afford a superbomb programme whilst its ally Britian ccame to the opposite conclusion. If OTL WW2 Germany had continued its superbomb programme it would have had to cut resources from something else.
You're right about the US economy being smaller in TL-191 though it's worth also considering that there was a lot of redundancy in the US atomic bomb program IOTL with the US developing two different types of atomic weapons. A US under more pressure and with less resources could have gotten by with a more streamlined program that only developed a single type of bomb.
On Germany though, there are a number of factors that would make TL-191 Germany far better situated to develop an atomic weapon than Nazi Germany ever was.
1) The TL-191 German Empire almost certainly has a much bigger economy than Nazi Germany did. (It won't be suffering from Nazi Germany's chronic economic mismanagement, and it fought a much less destructive First Great War, after which it was able to enjoy being the dominant economic power in Europe for a whole generation, so it's undoubtedly starting from a much stronger position);
2) TL-191 Tsarist Russia seems a much feebler foe than the OTL Soviet Union was. (The TL-191 Russians had a longer civil war than the Soviets did, and are starting out without the resources of Ukraine and the Baltic States. They also don't seem to have industrialized to anywhere near the same extent that Stalin pushed the Soviet Union, and of course the TL-191 Russians aren't getting Lend Lease from the United States.) If Russia is much weaker in TL-191, than the Germans won't need to donate anywhere near the same amount of resources to the Eastern Front that they did IOTL, which frees up resources for things like a real nuclear bomb program.
3) Somewhat related to point 2, but German goals on the Eastern Front in TL-191 are also much less grandiose than what the Nazis tried to do IOTL. The TL-191 Germans seem content to mostly just defend their own territory and only making very limited advances into Russian territory. (IIRC they don't even advance all the way to Petrograd.) Since they aren't trying to pull off anything nearly as big and complicated as Operation Barbarossa that will also free up a lot of resources for things like an atomic bomb program since its going to be a lot less resource intensive to defend your own territory rather than advance a thousand miles through enemy lands.
4) TL-191 Germany isn't doing the Holocaust. IOTL the Nazis wasted a staggering amount of resources on the Holocaust. (It was like a whole third front in the war for them.) Just by Germany not pursuing that monstrous program they will free up a tremendous amount of resources for other priorities.
. Instead I would look at say northern Alberta or the Newfoundland coast. A place with a low Canuck population, a fair distance from the USA and with unpleasant weather. Remember, TL_191 USA did not have much against Canadians but they really hated the CSA and Mormons.