Michael, in a word--yes. Your contractor might have let the wood get acclimated to your humidity, but he also might have enough experience to know that, with your location near a major body of water, and with relatively mild summer temperatures, enough people leave their window open in the summer. Your wood is probably 6% water right now. During the summer, it can easily pick up enough moisture from the air to be 12% or more. For every 3% change in moisture, your wood will move 1% in width. If you do the math, your wood will grow about 1/16" in width if you leave the windows open, or wash dishes or takes showers and the moisture content goes to 12%. Please note that this is not relative humidity. That is the ability of air to hold moisture. The percentages I'm giving are Equilibrium Moisture Content. This is calculated from the relative humidity and temperature as movement of both change the EMC.
You can check a lot of wood that is outside or open to outside conditions here and it will almost always be 12%. Right now, our temperature in the house is 68 degrees F., 19 C, and the relative humidity is 46% so we have an 8.4% EMC with the dog and kids going in and out often.
You may wonder about putting something on the floor to fill the gaps. If you do, the wood will expand and crush something. The power of water is tremendous. Roots can crack giant slabs of concrete, given enough time, correct? It's the force of the water in the roots that does this. So when wood wants to expand, it will. And if something is in the way, either the wood or the item will be crushed. Same thing if you put something in the gaps. If the wood can push it out, it will. If it can't move it, the wood will become crushed. The layers of cells that form the wood will just not be there. And they will not come back. When the same process happens next winter, the gaps will return.
Please note that this happens only with solid wood. The layering of plywood makes it work differently. And as far as the humidifier, 50% relative humidity is a pleasantly nice humidity to live in. My sinuses don't really like much less. But this is up to you. Your activities and the tightness and design of your house will really control this.