Going off 20 year old info from when I worked AA Load Control (weight-n-balance)...
The runway length at LGA wasn't the primary concern for the widebodies.
It was the "foot print" of the aircraft. I don't remember the details but think the issue was with the amount of weight being supported by the nose gear. It only has 2 wheels, vs the main gear that had 4 wheels each side, thus better weight distribution on the mains. I was told the pavement at LGA couldn't handle too much weight. So in addition to keeping the Load Plan balanced with respect to Center of Gravity, we also had to watch the weight on the nose gear. Sometimes by moving the load aft, but more often with a total weight restriction.
Over the years I recall LGA having traffic reductions and night-time curfews during summer due to "runway work". I often wonder if this work was to strengthen the pavement.