Usually in Final Fantasy, enemies get stronger as the game progresses for gameplay reasons. They provide a challenge for the player. However, in Final Fantasy X, a narrative reason exists for why fiends get progressively.
For example, we fight Seymour a total of four times. By this time, we really hate Seymour because he just won’t go away. And each battle becomes challenging to the point of tedium, because he’s much stronger than the last time. But if we keep defeating him, how does he come back stronger? It provides a new take on Nietzsche’s maxim:
“From life’s school of war: what does not kill me makes me stronger”
The reason is simple: he absorbs pyreflies every time he kills someone. Seymour becomes a menacing and much hated villain, as he dies he manages to take someone else with him. And he plans to take your characters with him.
What about the Fiends? Same concept can explain why they progressively get stronger along our journey. According to the Final Fantasy Wikia:
Consumed by sadness and rage, the spirits use pyreflies to assume a warped physical form.
Those who died in the war between Zanarkand and Bevelle, for example, would not make it to the Farplane. This explains why Fiends are very powerful in Zanarkand and Mt. Gagazet. Fiends would thus absorbed the UnSent, who exist as pyreflies. With no one around to Send them, the Fiends grow in strength.
Summoners Sent those who died in places such as Besaid, Kilika, Mi’ihen promptly. This would keep the Fiends in the area from becoming astronomically powerful. And also consider that:
Not all unsent are destined to be transformed into fiends instantly, some maintaining the forms they had in life because of a strong desire and sense of purpose. Many fiends are able to revert to the forms they had in life, only a few of them able to maintain the form for a long period of time and effectively gaining a second chance at life. Some unsent are able to transform into fiendish forms by absorbing Pyreflies into their bodies.
Besaid, Kilika, Mi’ihen are relatively new areas. Those who may have died may not have had time for Fiends to absorb them.