In a mere 20 city blocks, SF has a plethora of totally different developers building a wide range of uses within a plethora of 350-400+' towers or mega projects (Transbay Terminal and SFMOMA expansion).
NYC will always have more due to its sheer size (not quite 100x more though

), but I don't think even NYC can lay claim to as many different developers putting up as many brand new office, resi and mixed-use towers in as small an area as SF. Not to mention, this area in SF is still plagued by surface lots and low-rises, so it's a totally new look and redevelopment that will transform the skyline. The two big mega projects in Manhattan are Hudson Yards and WTC Redevelopment. The towers are massive. But as a percentage of what's in the skyline already, and in terms of what's surrounding the developments, and in terms of # of developers working on these projects (not to mention WTC is basically a huge office development instead of a mix of uses), they aren't necessarily comparable, even if they are developments that will end up putting more square footage on the ground as a whole.
In some sense, the area of the continuous SF skyline is nearly doubling (especially when viewed from east or west), and it's not like the SF skyline is small - it's actually one of the largest in the country.
Miami has a fair amount of projects going up in a confined area, but fewer individual developers and less of a mix of uses (mostly resi there). Seattle has its moments, but the buildings are much shorter.