It's more than likely because you're too wound up or nervous that's making you "tighter" just by being tense and as a result, clenched up.
Although it is possible that sometimes people are just physically incompatible and a guy with an extremely large penis and a very tiny, petite woman might just have a difference in their bodies that is difficult to overcome.
I agree with Shy though. Hold off on sex until you're more comfortable and maybe, for lack of a better way of putting it, more "open" to it.
OIther than that, making sure you're aroused, well-lubricated, and emotionally ready can help this a long way. Some women find it easier if they are on top so they can set the pace and the depth of the action to prevent pain.
No, it can't be too tight. The vagina stretches, that's how you are able to give birth later on. You have to make sure you are not tense/stressed or the vagina muscles will be tense making it hard for penetration. Make sure there is enough foreplay for you to be fully turned on and ready. Also keep open communication with your boyfriend, if you're nervous talk to him about it. Make sure you are really ready for this. Good luck.
Yes, a vagina can be too tight; and a penis can be too large. So, the question might also be... what can your boyfriend do to make penetration easier on you? He can penetrate you slowly and gently and avoid going all the way in... until you no longer feel so tight.
Oh... and, adding the use of a lubricant during intercourse helps too.