Why might the Tripp Trapp® Baby Set² feel tight for my baby before reaching the maximum advertised age of 3 years?

If you notice the leg space of the plastic Tripp Trapp® Baby Set² is too tight for your child, ensure the Tripp Trapp® chair is correctly adjusted (with the seat plate installed per the Tripp Trapp® user guide) and the Baby Set is fitted according to the instructions in the Baby Set user guide.

Both user guides can be found on stokke.com. 

In general, the Tripp Trapp® Baby Set² adheres to stringent testing and meets international safety standards for high chairs (for use between 6 and 36 months). It is designed to accommodate the average child. Stokke complies with safety regulations which set a maximum size for allowed leg openings. 

Of course, it may well happen in some cases that a child grows faster than the norm and outgrows the Baby Set before it reaches the maximum age advertised.