I find cycle shoe sizing a bit hard to work out and I am wondering if I am buying too large.

I am 5ft 7 and wear a size 8.5UK (sometimes a 9) or european 39.

Over the last few years I have brought-

2 pairs of Nikes in size 42 men's

1 pair of Lakes in 42 men's.

I tried a pair of Sidis on this weekend and once again they were 42 men's.

I can understand going up one size, but going up 3 sizes!

With the Sidis they felt great and I had a thumb width between my big toe and the end of the shoe. My foot is wide but not super wide. The Sidis were the Dragon MTB shoes so weren't mega width. Am I finding the larger size to get the right width? Would I be better to try and find a shoe with the correct width but smaller?

BTW I get numb left foot in all my shoes and I have put this down to a large vein on the top which probably gets a bit crushed buy the straps, so I try and loosen that one up a bit.