The automatic shipping calculators for UPS, USPS and Fedex impose limits on the weights of products being shipped. The limit on a single package varies by carrier:
UPS: 150 lbs
USPS: 70 lbs
Fedex: 150 lbs
So if a customer tries to check out, and any single item is heavier than the maximum, that carrier will not be available to the customer.
For this reason, if you have heavy products, you may want to add Custom Shipping methods as well as, or instead of, the automatic UPS, USPS and Fedex options. For help with that, check out the 'Custom Shipping Rates' Knowledge Base article.