Google considers a Heavy Ad when the ad uses more than 4 megabytes of network bandwidth, or uses the main thread for more than 15 seconds in any 30 second window, or uses the main thread for more than 60 seconds in total.
The user must have not interacted with the ad, for example, has not tapped or clicked it, to be considered a Heavy Ad.
Chrome is blocking Heavy Ads since July, 14, 2020. The Heavy Ad Intervention was released on desktop and on android, in the version 84. The feature is enabled by default.