yes that a problem i was aware of but i didnt really have any good idea on how to fix it maybe killing the player if he stays the same size too long ? and the i didnt think the perspective was so bad :
I think the first step should be to make it more clear that you get higher score, and making it feel good to actually fill up the holes.
But there would still be people who optimise away the fun, so maybe a timer that kills you after holding the same size for too long. But then you could just release it and then hold it again. I guess you could check how many holes you had passed through with the same size.
Also maybe killing the player isn't necessary, there could also be some kind of score penalty. But either way it would need to be communicated clearly.
Another possible solution would be returning to default size after each obstacle. Then you might also make it so you don't need to hold the keys, but could just tap them. Which would be an accessibility improvement.
If you decide to develop this further I think it would be really nice to play with controller.