Fish has too much volume to sit on?

After trying an 8 foot surfboard for a few months, I wanted to get my own that I can easily store and transport (condo and a sedan) so I ended up getting a 6 foot softtop fish with 40 liter volume. (https://www.southbayboardco.com/product ... 0507578456).
I can really paddle well with this and I love how I can nose dive with this board. It's SUPER light and I haven't had the chance to attempt a wave with it yet since I have injured ribs (over did my muscles so trying not to make it worse).
I did try and sit on the board but I can't stay on top of it (but I do know it is more difficult being a shorter board) and tend to tip over. It is my first time trying a shorter board and for the most part I've been trying to sit on it like this:

Every time I did this I would have to hug the board with my legs and eventually flip sideways after a couple of seconds.
I know it's supposed to be more like this:

I remember trying to attempt the second position a couple of times but I REALLY struggled to get it to sink like that and often I would nosedip and flip forward.
However, I see surfers do the first position just fine:

If I were to try that my board tries to shoot up and forward and won't want to stay sinking like shown in the pic above...
I'm going to try doing this again tomorrow with that second position, but I think the board is way too buoyant to even stay down. The board is extremely light being foam softtop and super buoyant at 40 L. Do you guys think the board is causing the issue? Will have to try and rent a hardshell shortboard with less volume if I can find a store that does...
*edit I'm 5'10 and 185 lbs.
I can really paddle well with this and I love how I can nose dive with this board. It's SUPER light and I haven't had the chance to attempt a wave with it yet since I have injured ribs (over did my muscles so trying not to make it worse).
I did try and sit on the board but I can't stay on top of it (but I do know it is more difficult being a shorter board) and tend to tip over. It is my first time trying a shorter board and for the most part I've been trying to sit on it like this:

Every time I did this I would have to hug the board with my legs and eventually flip sideways after a couple of seconds.
I know it's supposed to be more like this:

I remember trying to attempt the second position a couple of times but I REALLY struggled to get it to sink like that and often I would nosedip and flip forward.
However, I see surfers do the first position just fine:

If I were to try that my board tries to shoot up and forward and won't want to stay sinking like shown in the pic above...
I'm going to try doing this again tomorrow with that second position, but I think the board is way too buoyant to even stay down. The board is extremely light being foam softtop and super buoyant at 40 L. Do you guys think the board is causing the issue? Will have to try and rent a hardshell shortboard with less volume if I can find a store that does...
*edit I'm 5'10 and 185 lbs.