Seems to be common to have reduced weight capacity for 16" wheels perhaps the limitation is the wheels themselves or just the fact the bikes are even more compact however for larger 20" wheels weight capacity is often comfortably in the 115-125kg area by many manufacturers. I tried to find information on the Dahon Curl but all I could find was a kickstarter campaign page, intended weight capacity and actual certified weight capacity are 2 different things. I couldn't find the Curl as a commercial product. The original Dahon and Tern bikes may be were engineered to be at the 120kg weight capacity approx but after the certification process were forced to state a lower amount.

I'd also point out the certification process probably only allows for a relatively short lifespan in their testing could be 3, 5 or 7 years expected lifespan. While a steel frame bike may remain at the same strength for decades an aluminium frame will likely reduce in strength over the years unless designed so strong and rigid it will not flex which is not normally the case with aluminium bikes. So a Brompton of 110kg today could be the same in 15 years but a Dahon of 105kg could be in the lower 90's of KG capacity after the same period. That is a complete estimate I have no way of knowing the reduction in strength but it is meant to be significant but unsure what that is in numbers. Many aeroplanes used to have a replacement regime for aluminium parts at regular intervals but advancements in scanning metals for fatigue plus a movement towards titanium and advanced composites in aeroplanes has removed aluminium from some critical areas. There is also the issue that if you exceed the weight limit of a steel bike the reduction in strength of the frame may be slow with the fatigue only gradually reducing the strength of the frame but with aluminium the same situation would result in a rapid reduction in strength of the frame. I'm only making that point because comparing a steel and aluminium bike of the same weight capacity is not equal over time.

I don't have access to the full 4210 standard. It is possible that they allow for aluminium fatigue and the claimed weight capacity in reality is actually the figure they calculate after 3 years of use. Perhaps someone in the trade who does could state whether that standard allows for fatigue and lifespan of frames in their testing processes. I suspect they don't and treat aluminium and steel pretty much the same and do a series of strength tests using weights mounted and hitting the frame, drop tests and bending tests to simulate actual use under controlled repeatable conditions.