If your objection is that the building is going to take up too much of the parcel and that there won't be much grass left, say so. Many boards are sympathetic to those who want a green community. If the zoning change sought would block light going into your property, say so. The original zoning laws in New York of 1916 were designed to prevent a loss of light, and most zoning boards are conscious of this. If your objections are not based on facts, but on opinions, you may wish to consider having expert testimony as part of your objection.
Objectors can call witnesses the same as a proponent can.
If you have environmental concerns, express them. More and more zoning boards are concerned with ground coverage and impervious surfaces that deny the groundwater the ability to be renewed. If there are possible hazardous materials, bring this to the board's attention. Because it has already been done doesn't mean it can, or should, be done again. A picture can be worth a thousand words of testimony.