Recent experimental tests have shown that RBS connections incorporating Jumbo specimens meet the current seismic design qualification protocols, allowing to further extend the current seismic provisions for prequalified steel connections with possible applications of heavy steel sections beyond their current use in ultra-tall buildings. The experimental results and observations described in this paper enabled a better understanding of the structural behaviour of RBS connections made of heavy structural sections for application in seismic regions. How-ever, the results indicate that geometrical and material effects need to be carefully considered when designing welded RBS connections incorporating large steel profiles. To better interpret the experimental results, extensive detailed non-linear finite element simulations are conduct-ed on the entire series of tests, comprising of three large-scale specimens with distinct sizes. The analyses intend to clarify the scale effects that influence the performance of these connections, both at material and geometric level, and particularly to understand the balance in deformation between the column panel zones and the reduced beam section and level of stress within the main connection components. It is shown that the numerical models for all three specimens reproduce accurately the overall load-deformation and moment-rotation time history.