I asked Vauxhall about this ages ago...apparently the X16XEL is designed for lugging stuff around (was used in Astras and Vectras - obviously heavier than yer Corsas!). They said there were small head differences too, probably to make it more efficient and/or shift the torque range.
The two engines have different part numbers on the EPC for block and head, so maybe more internal differences too.
pistons are same for all 1600 variations, but as phazer hinted the heads are different. the X16XEL shares the same head as its smaller 1.4 brother. whereas the higher power 1600's X16XE and C16XE share the same head.
Must be flow characteristics, or maybe compression