Reason is not the size of trunking - but cable separation :-)
Eg, installation Method M1 for cables on surface... o 1 Dia separation, N= 2, Cg = 0.94 o 0 Dia separation, N= 2, Cg = 0.85 -- cables touching on surface
Cable bunching between joists requires Cg = 0.90.
When we ran 8 cables they were arranged in 4 pairs with one of each pair being 1.5mm 6A (Lights Smoke Alarm). Thus 30% cable load allows N=2 cable grouping to be disregarded (N=1).
Each pair ran through a P-clip/screw/wallplug to maintain separation >2D largest cable major axis from the next pair. Bricks too hard for clips, mortar too weak for clips. That way derating was kept to a minimum, buried direct M2.
Physically very tedious, with hindsight it may seem better to use a single 6242Y or SWA to a mini-CU instead. Even
10mm SWA fits thro a standard 20mm backbox hole if you want to mark a route outside zones, min bend radius is handled via bending up the wall on internal corners. Typical problem is you end up routing N cables "back" along the route you took to where needed - so rejected.Where cables are grouped for short distances you can ignore grouping - basically as you would for a joist width. However it may be wise to assume bunching Cg of 0.90, for domestic usage this should rarely be a problem.
Industrially they (literally) throw CSA at the problem if in supersized trunking or use cable trays accordingly. Cable trays often carry SWA which is XLPE, so absorbs cable derating problems re higher current rating. It is not practical to do that with say XLPE 6242B domestically because the 90oC figure does not apply to fittings. Plus you can not use that figure & route by PVC cables.
The FP200/400 etc concentric cables are popular as they remove the need for armour in some circumstances, so are quicker to fit (altho not cheaper). Unfortunately it seems nearly all are 70oC rated vs SWA XLPE so as a result the "grouping problem absorption" is lost, but since they are smaller than SWA separation increases.
With planning you can avoid most grouping problems. However 24-way MK Grid boxes must be *fun* to do.