If you think that embryos and fetuses have moral value, then abortion becomes a very important issue in terms of scale.
This might not be the case if you have a narrow person-affecting view so that whether A or B is born doesn’t matter, even if one would be substantially better off than the other (see my answer on the nonidentity problem). In that case, the fetuses that don’t yet exist (or those that won’t exist until after some point) might not matter, because which ones would come to exist could be sensitive to your actions (think butterfly effect). Then, the scale of the problem is restricted to the fetuses whose identities are already determined, and you might be too late to help almost all of them.
Same conclusion with presentist views, so that only those that currently exist matter.
This might not be the case if you have a narrow person-affecting view so that whether A or B is born doesn’t matter, even if one would be substantially better off than the other (see my answer on the nonidentity problem). In that case, the fetuses that don’t yet exist (or those that won’t exist until after some point) might not matter, because which ones would come to exist could be sensitive to your actions (think butterfly effect). Then, the scale of the problem is restricted to the fetuses whose identities are already determined, and you might be too late to help almost all of them.
Same conclusion with presentist views, so that only those that currently exist matter.
EDIT: Larks made the same point.