Tooth decay generally becomes more difficult to treat once it progresses and causes increasing loss of tooth structure.
A cavity that may initially require a relatively small filling can potentially become larger and eventually require more extensive treatment.
Depending on how far the problem progresses, treatment could potentially involve:
Small cavity → Larger filling → Crown → Root canal treatment and crown → Extraction
Not every cavity will follow this exact progression, and the appropriate treatment varies by tooth. The important point is that early detection can often allow for more conservative treatment.