Ramo vs. ramillete
The words 'ramo' and 'ramillete' both refer to groups of flowers, but differ in size, formality, and context.
Ramo
A bouquet or bunch of flowers. This term is often used for larger arrangements, such as those found at weddings or funerals.
El novio le regaló a su prometida un hermoso ramo de rosas.
(The groom gave his bride a beautiful bouquet of roses.)
Los ramos de flores adornan el salón en el funeral.
(Bouquets of flowers decorate the hall at the funeral.)
Ramillete
Literally a 'small branch', this term typically refers to a small bunch or cluster of flowers. It may also have informal or affectionate connotations.
La niña recogió un pequeño ramillete de margaritas silvestres.
(The little girl picked up a small bunch of wild daisies.)
Él siempre le regala a su esposa un ramillete de flores de su jardín.
(He always gifts his wife a small bunch of flowers from his garden.)
Summary
'Ramo' and 'ramillete' both refer to collections of flowers, but 'ramo' often implies a larger, more formal arrangement like a bouquet, whereas 'ramillete' indicates a smaller, perhaps more casual collection of flowers.