I have the same issue. I love the stroller for it's one-handed set-up and fold-up, and the infant seat worked great. Honestly, I never strapped my daughter into the regular stroller seat. Thus far, I haven't had a need to.
The highchair is a different issue. Because of the hard saddle under the tray, I also never used the straps. But now that my daughter is old enough to be at the table, I don't use the tray and have to strap her in so I can let her sit at the table with me. McKenna is 14 mos, 20.5 lbs. and 26 inches tall and the straps are STILL too long, even at the tightest.
I do love the different height adjustments, though, because we have had a few occasions when we ate in another room (holidays and such) and I could lower the chair to accomodate my seat. Also it was necessary to lower the chair for it to fit under the table.
Another thing I hate about it is that the straps cannot be removed for cleaning. She recently got very ill and threw up all over the chair and it was a pain getting the straps clean while attached to the chair.
The instructions are very poor. It seems that they were originally written in a foreign language and didn't translate well. However, I have gotten very friendly service from the company. You can find their number on the company's website.
However, if you can return the highchair, I recommend it, unless the other features are worth making do.
HTH
C.