There are a couple of occasions where I've been on really packed services in Europe.
One was where in Paris and I was trying to get between Gare Du Nord and Charles de Gaulle Airport on the RER leaving at around 08:45. The train was heaving for the first couple of stops but by the time it got to the Airport the train was 3/4s empty.
I haven't used that service. But I do remember a busy train in the South of France. This was because of a cruise ship in Villefranche, and there were three or four people standing in the vestibule on the train from nice to Ventimille. Like your experience, though, the trainw as pretty empty for most of the route, as almost all the cruisers alighted at Monte Carlo!
It still didn't come anywhere near what I've seen on Southampton - Bristol or Shrewsbury - Manchester though!
The other was getting between Barcelona Airport and Barcelona Sants. Although, there were engineering works meaning there was no through service and the Airport trains were running as shuttles to El Prat instead of to Barcelona and you had to pick up another train from there, so there were probably two train loads of passengers on the trains that were running between El Prat and Barcelona.
I have done that one, albeit not at a time when engineering works were disrupting things! Arrive din the evening at about 1830, got stuck in the barriers at the airport station, and found the train to the cnetre busy but with plenty of room.
I think that the larger loading gauge certainly helps to reduce the sense of overcrowding compared to the UK, as the traisn in Europe seem to be far more spacious even when they are pretty full.
On the way back it was mid morning, and the airport train was pretty quiet. Less busy, I thought, than in the other direction, but perhaps not surpising given the time of day.
But back to your original question. I have no experience of this line or the trains on it, but I imagine it is regional type service? If so, then I'd be prepared to wait for the next train if the one I intended to catch seemed really busy. If that is possible, of course.