why doesnt apple reveal data size of iphone updates?

They do.

When updating both iTunes and the device itself will show the update size:

DanaH676, Were you refering to updates in apps or IOS? Phil0124 is correct in the IOS updates, but after IOS 11 the size of app updates stopped showing as they did in IOS 10. Phil0124 do you have answer for the app size not showing anymore? It would be nice to know if one is using cellular for the download. Thanks.

You can still see the size of the app update. Go to the App Store. Tap on Updates. Tap on the app that you want to update. Swipe down and you will see the update size under the Information heading which is under the ratings section.

EDIT : Sorry for the very late and redundant response. I had it all typed up, walked away for bout 10 minutes, came back and then finally hit reply.

Demo wrote:

You can still see the size of the app update. Go to the App Store. Tap on Updates. Tap on the app that you want to update. Swipe down and you will see the update size under the Information heading which is under the ratings section.

EDIT : Sorry for the very late and redundant response. I had it all typed up, walked away for bout 10 minutes, came back and then finally hit reply.

😉 No worries. It's happened to me on more than 1 occasion.

Tap on the App you wish to update, and then scroll down to the "Information" section:

That is the size of the download for the update:

Ignore the red ellipse over the Location requirement.

Phil0134, WoW I never new. Learn something everyday. Thank-you😁. GeeZ06

Thank-you , All good.