No they haven't reduced the amount of copper, they wouldn't be 25mm if they had.
They have increased the number of strands therfore made the cable more flexible.
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That is not the correct soloution to the problem, either a reduction of the load or a larger supply is required, changing the fuse for an MCB will not solve the problem.
By fitting an MCB in place of the fuse you have made the discrimination (selectivity) worse. Also the regular overloading of the MCB will cause the thermal element in it to suffer damage and probably cause the MCB to trip more readily or cause it to fail completely.
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I dont really see a need for it, the overall flexibility of the cable won't be helped as this will be governed by the steel armour.
As long as you use decent switchgear and good installation practices then there is normally no issue working with 25mm or 35mm (50mm and up is 19 strand)