How to report potential abuse in Aptoide without becoming a member?
First: Different sizes do not necessarily mean the bigger one is infected with malware. Or rather, that might depend on your definition of malware: F-Droid compile .apk files themselves, and while doing that, often exclude proprietary parts – such as ad modules.

Second: Aptoide has security checks running on their .apk files. You should find a shield in the app description: Green = checked and OK, Yellow = checked and something suspicious found, Red = checked and infection found1. You can also find details on the checks, where e.g. signatures are compared with other markets (usually Google Play).

Third: Different repositories at Aptoide have a different trust-level. For example, the repository named "Apps" is manually curated by Aptoide staff, and hence quite trustable.

Forth: You can contact Aptoide via a form on their website. You'll have to reveal an email address for that, so they can contact you back.

Fifth: For more details, please see How safe is it to use Aptoide?. Especially check this answer from Paulo (co-founder of Aptoide) and my own answer.



1You will never find the red shield, though – that's only visible for staff. At the same time a red shield is attached, the app is made unavailable for the public.