Mulgrew & Picardo openly admit and never denied going to the producers to have more screen time and larger roles in the series. It's comon knowledge that Mulgrew felt threatened by the introduction of Ryan and how they used Seven instead of Janeway to promote Voyager. Mulgrew felt as if she were being replaced as the main draw to the series. Based on those facts, neither Mulgrew nor Picardo felt as you do in regards to the amount of screen time their characters would receive.
Doesn't matter to me how it happened really. I like season 4-7 the most and mainly because of them. I'm glad they pushed for more time. Good for them. It worked out well for me since I loved their character.