My larger line is a favourite English style. It has an outer braid, an inner braid and a core.
I cut two inches of the core, one inch off the inner brais and then pull the inner through from the small line and melt the two small cores together. This must be a very tidy small join so no hard spot.
I then pull small lines out over the join and melt the frayed end voer so it is smoth, wont pull back over join but has no large hard spot. I then pull braided inner of larger line over and then pull out two of the braided sections back to the joing of the two core lines and cut and remove. This ensures there is no swelling in the finnished sheet. I pull the large outer over the lot and whip it it good.
No swelling, very strong, nop creep and only a half inch or smaller hard spot where the whipping is.