Regarding the current situation, it's important to note that the block size of Bitcoin is actually limited to 1MB, not 4MB. However, the implementation of SegWit allows for more transaction capacity by separating some of the transaction data. This has helped alleviate some congestion on the network.

A 300-byte transaction is 300 bytes on-disk and over-the-wire. Segwit just counts those bytes differently toward the maximum block size of 4M weight units.