Cycle shoes sizing- am I buying too large?

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.

It is worth noting that with good shoes mens and womens are made on a different last. Mens feet are (in general) wider through the whole foot, whereas women tend to have a narrower heel and broader forefoot.
Another point is that mens sizes are sometimes upsized for sales reasons, but womens sizes are downsized for sales reasons. Sizes can only ever be an approximation of actual size.

I wear a size 8 in UK sizes and a mens 39 in road shoes.

If they fit your feet, it doesn't matter what the number says. I don't buy shoes before trying them on, but some people do. I assume you can return them if they don't fit, but that's such a pain. Better to try them on before you buy.

I think you have to go with what feels right -- it seems there's some fudge factor both in the US/Eur size conversion charts and in the men's/women's size conversion charts, so if you're buying a shoe in men's/"unisex" Eur sizing, god help you ;).

I wear a 7.5/8 B in American women's street shoes. I'm usually buy 39 in cycling shoes, whether men's or women's. Unscientifically, it seems that many women on TE who wear a similar street shoe to me wear a slightly larger cycling shoe... so you've just got to try them on and see what feels right.