Something that is substantial is (1) of considerable size or importance, (2) solidly built, (3) ample, or (4) well-to-do. Substantive means of or relating to substance, where substance means meaning. So substantive is often synonymous with meaningful, while substantial is usually synonymous with large or important.
Examples
Substantive
Richmond blames term limits for nurturing an atmosphere dominated by spin and talking points over substantive debate and the soaring oratory of decades past. [Miami Herald]
And that’s what’s interesting about the divide: It is as much stylistic as it is substantive. [Los Angeles Times]
This is a pretty substantive pact involving two treaties, one on joint military exercises and defence procurement, the other on nuclear co-operation. [Financial Times]
The substantive objection to raising taxes is that it might slow the still sluggish economic recovery. [The New Republic]
Substantial
That’s a no-brainer; it’s a smart plan with substantial community support. [Dallas Morning News]
The married banker, paid a substantial six figure sum, began the illicit affair before the credit crunch erupted and plunged the country into recession. [The Sun]
A seven-year-long period without substantial rainfall, along with two decades of conflict, have put the African country on the brink of a catastrophe. [Toronto Sun]