Ok but you must be on iOS 11.3 Beta, right? Up to 11.2.5 the file sizes aren't shown on updates page.
A question to those more knowledgable about the Sparkasse's background: All banks (usually county or town wide) are independent, but their infrastructure is basically identical, right?
As far as I know the backend of the online banking system is identical, the card issuer is (always?) BayernCard Services for MasterCard and SV (or whatever they're called) for the Girocard. So if the Sparkasse was able to align themselves in the whole of Germany to prepare for contactless payments (see them rolling out Girocard Kontaktlos from July 2017 at all Sparkassen, Android Wallet within the Sparkasse app as they have announced from July this year), then maybe it is on the cards. To be honest, there is no real definitive payment option for Android, so it would make sense for the Sparkasse to develop their Wallet within the app. That could also be transferred to iOS. Keep your card within Sparkasse+ and add to your iOS Wallet...