The size of .ipa files can vary, significantly, based on the options used. The .ipa file size does not directly correlate to the size of the app store download or the size of the installed app. It's normal for a development build to produce a much smaller .ipa than a distribution build for the same app.
"Important: App Store submission .ipa files are not representative of the actual size of an app on a device, and should not be used for this examination. Only measure using .ipa files produced by following the steps in Getting an App Size Report."
If size is a concern in your project, begin by generating a thinning report and inspect your .ipa as described in the following Apple doc:
Ionic Appflow uses standard Xcode tooling to package iOS apps. The resulting .ipa file will be similar in size and composition to a build run locally using the same Xcode version. Ionic support cannot assist with app size evaluation or thinning techniques.