When looking for new opportunities, a less cost-effective (in terms of social good per dollar spent) opportunity that is more scalable (in terms of total dollars that can be spent to achieve the target cost-effectiveness) can sometimes be more exciting and more helpful to the overall EA portfolio than a more cost-effective but less scalable opportunity.
In the ITC framework, this is captured by diminishing returns. To optimally allocate resources, you give your next dollar to the intervention with the highest marginal utility per dollar. This means funding the low-scale intervention until its MU/$ is below that of the high-scale intervention, and then switching to allocating your next dollar to the high-scale intervention.
Restating your point: if you have a huge budget, then you need to have scalable opportunities (ie. with low diminishing returns) in order to spend your whole budget. There might be a bunch of small interventions (ie. fully funding them would use up 0.0000001% of your budget) with the highest MU/$, but if there are transaction costs to identifying and funding them, it could be optimal to ignore them and focus on more scalable interventions.
In the ITC framework, this is captured by diminishing returns. To optimally allocate resources, you give your next dollar to the intervention with the highest marginal utility per dollar. This means funding the low-scale intervention until its MU/$ is below that of the high-scale intervention, and then switching to allocating your next dollar to the high-scale intervention.
Restating your point: if you have a huge budget, then you need to have scalable opportunities (ie. with low diminishing returns) in order to spend your whole budget. There might be a bunch of small interventions (ie. fully funding them would use up 0.0000001% of your budget) with the highest MU/$, but if there are transaction costs to identifying and funding them, it could be optimal to ignore them and focus on more scalable interventions.