A small piece is a real offer
A toddler portion can look almost too small to an adult. A few bites of a food they know, plus a little of the new one, is a full offer.
Small is kinder
A huge serving can make them stop before they start. Small is easier to face, and you can always give more of the food they want.
Think of a few spoons of rice and a teaspoon of the new food. That is a real dinner for a young child, not a sample you must apologise for.
When they want more
If they finish the familiar food and look for more, give more of that food. The new food can stay the small piece.
Refilling the food they accept teaches them that the plate is safe. It does not teach them to hold out.
What a big plate does
A piled plate often gets pushed away whole. Then nobody eats, and the meal becomes a rescue.
Start small. Add food in front of them if they are still hungry. Let them see the adding.
A sentence you can say
“There is a little here. I can give you more rice.”