Originally, Bitcoin blocks had a hard-coded limit of 1 MB. This restriction helps prevent network spam and controls resource usage for node operators. However, with the activation of Segregated Witness (BIP 141), the concept of ‘block weight’ was introduced, allowing some transactions (particularly those using SegWit) to occupy additional space beyond the nominal 1 MB limit.

In practice, this means that post-SegWit blocks can exceed 1 MB in raw data size, often reaching around 1.3-1.5 MB on average. The new measure, called the ‘block weight limit,’ tops out at 4 million weight units. The broader goal is to balance transaction throughput with decentralization, ensuring full nodes remain accessible to the widest range of participants.