It's kind of both but mostly the latter. The amount of data per block increases pretty massively as you approach the current block. It's common for more modern blocks to have several thousand transactions for every single block, where this was not the case in the early days.
Discussion
Fair. After I had synced fully I was very silly and executed a 'Repair' command on my node that essentially reverified UTXOs. This process also hit a significant slowdown at about 80% to completion, which I'd wager correlates with the spike in UTXOs in early 2023.