Who has the highest population?

I've noticed this in other areas too. The highest percentage does not always produce the highest boost. Size, location and how many can produce an overlapping result all play a part.

 Exactly, being somewhat of a Specialization freak you can't take anything for granted. The raw numbers don't tell the whole story two features with the same area of influence (AOI) and same boost values may provide different population boosts. Some general rules of thumb everything else being equal the building with the smallest footprint has the potential to produce a greater boost, this is because the AOI is measure from the center of the footprint not edge reducing the number of buildings impacted so larger buildings will often provide lower real boosts. Specialization Line overlap also seems to also play a factor the greatest boost seems to occur in buildings that aren't serviced by a particular line and impact drops incrementally for each Specialty building of the same line after that. Specialty Buildings with either rectangular AOIs or footprints may give dramatically different results when rotated 90 degrees (see the images of the libraries) or in some instances a feature with seeming lower numbers will provide greater boost because of the way the rectangular foot print is oriented relative to the footprint in certain locations (see the images of the parks). Also remember for a building to receive a boost it's footprint must be intersected by the AOI of the Specialty Building it is not enough to just touch it. This is why some times a slight shift in position can make a huge impact on actual results. If you see a saw toothed edge on opposite sides of the green AOI overlay when placing or selecting a Specialty you know you have squeezed every bit of impact out of that building.

3 image limit kept me from showing parks but with parks especially spending more does not guarantee a greater boost, test before buying, many of the less expensive parks will provide greater boost in the right location.