Why Sweden still stays outside the treaty

Would Sweden be invited once Soviet naval construction increased?

No; Sweden could build its own coastal defenses, but it was not considered a major naval power requiring treaty membership.

Sweden could answer Soviet construction with stronger coastal-defence ships without accepting treaty restrictions. Its armed neutrality and naval industry mattered within the Baltic. The fleet was still too small to justify a seat in a great-power tonnage bargain. Stockholm could protect its regional interests through independent building rather than conference membership.