Monero is less scalable because you have to store all tx outputs (key images) instead of just unspent tx outputs, since Monero does not know which outputs are fully spent (for larger ring sizes). And these must all be stored in fast random access memory for double spend prevention. That’s an ever-growing pile of data!

While in Bitcoin you can offload older utxos to a slower memory, because the access pattern is “utxos die young”. Also, utxo set grows much slower or even can be stable (“1 utxo per person”) while tx volume grows forever.

Bitcoin scales way better than Monero or Zcash (that has the same requirement to store all “nullifiers” to prevent double spending).