The majority of Cortland's 555 series feature their proprietary mono-core as depicted here. Vastly different than the more conventional braided centers, the mono-core lines tend to be somewhat larger in diameter than their more traditional counterparts. Consequently, a mono-core 555 will occupy more space on the spool resulting in the loss some backing.
However, not all 555 lines feature the mono-core center. The 555 Dyna-Tip is just such an example. It features a low memory nylon braided core that has little effect on the diameter of the line.
It may be important to note that the increased diameter of the mono-core lines does not imply that they are "heavier" than their counterparts. The fact is the first 30-feet of a 5-weight line will weigh between 134 and 146 grains regardless of the core. That's the AFTMA standard.