Multiple insert performance trick or performance misunderstanding?

When I need to insert a few hundred or thousand things in
a table from a 3-tier application, it seems I'm much better
off creating a big string of semicolon separated insert
statements rather than sending them one at a time - even
when I use the obvious things like wrapping the statements
in a transaction and using the library's prepared statements.

I tried both Ruby/DBI and C#/Npgsql; and in both cases
sets of inserts that took 3 seconds when run individually
took about 0.7 seconds when concatenated together.

Is it expected that I'd be better off sending big
concatenated strings like
"insert into tbl (c1,c2) values (v1,v2);insert into tbl (c1,c2) values (v3,v4);..."
instead of sending them one at a time?

db.ExecuteSQL("BEGIN");
sql = new System.Text.StringBulder(10000);
for ([a lot of data elements]) {
sql.Append(
"insert into user_point_features (col1,col2)"+
" values (" +obj.val1 +","+obj.val2+");"
);
}
db.ExecuteSQL(sql.ToString());
db.ExecuteSQL("COMMIT");